How long it will take for the subconscious to accept and act upon suggestions depends on how deep seated the issue is that you are trying to change and how effective the suggestions are.  Sometimes the change can happen quickly, sometimes it takes longer.

It is very hard for the conscious mind to understand how the subconscious mind works.  For the conscious mind can not accept the fact that if you visualize in your mind that you are opening a door, it is the same as physically opening the door.  To the conscious mind the act of visualizing opening the door is pretending and it did not actually happen. However the subconscious mind does not know the difference, each act is just as real to the subconscious mind. as this is how the subconscious mind receives its information, through thoughts and pictures, not actions.

Hypnosis is extremely effective if you can visualize in your mind actually accomplishing the positive suggestion you are sending to the subconscious mind.  For instance, if your positive suggestion is I will feel full faster and eat less, imagine yourself doing just that.
When you take control of your mind and plant suggestions for change you can become free of bad habits, lose weight, or stop smoking because those suggestions become part of your internal programming.

If your mind can believe it, you can achieve it

Often times you will hear terms being discussed in hypnosis, like conscious mind, subconscious mind and/or unconscious mind, and you may be wandering exactly what these terms mean.  I think it is imperative to understand how each of these parts of the mind works in order to understand hypnosis.  First however I want you to understand that I am not a psychologist, so if you are expecting a technical explanation you are not going to get it here. That I apologise to some for, but I tend to think on much simpler terms.

We operate daily on several different levels of the mind.  It's quite amazing really, and the more you understand the way the mind functions the easier it is to make change.  We basically operate at three main levels although there is a fourth that is not talked about as much, which is like the barrier, or the force field, or the sorter, however you want to refer to it as, which will become much easier to understand once we define each part.

The unconscious mind

The unconscious part of the mind, is the part of the mind that functions without us having to think about something in order for it to function.  This is the part of the mind that controls all of our bodily functions that keeps us alive.  It controls the heart beating, the blood flowing, the need to eat, rid the body of wastes and so on. All these things happen without us having to have a basic understanding of anatomy and how each of these parts function in order to get them to work. It is comparable with the pre-program on your computer system, the program behind all the other programs, that makes all the other programs have the ability to function. 

This is the deepest level of the mind.  Even though this is the deepest most automatic part of our mind, it can be altered or affected to react in abnormal ways.  Such as when you are under allot of stress, or anxious feeling, or ill, sometimes those feelings are strong enough to actually reach into the unconscious and impact some of the natural rhythms of that part of the mind.  For instance, when you are feeling really anxious, sometimes it can affect the heart rate, causing it to speed up, or perspiration, or any number of other functions controlled by the unconscious.  

Just as there are unhealthy aspects of our lives that can reach into this part of our mind and alter the normal functions, there are healthier ways to reach it as well.  For instance, daydreaming, where your mind wanders off, your daily stresses are forgotten for a short time and your body begins to relax and send a calming sensation to the unconscious.  Meditation is another way to learn how to create change in this part of the mind, changes like slowing heart rate, deeper breathing, and from what I have read and understand changes that most would think impossible to achieve, like faster healing rates within the body itself, and many other changes.

The subconscious mind 

I personally like to refer to this part of the mind as the bookstore of the mind.  Some refer to it as the library, or others as the hard drive. This subconscious mind contains all the information you have seen, heard, felt. touched. smelled, experienced and learned throughout the beginning of your existence.

This is also the part of the mind that houses all our beliefs, our moralities, our perceptions of ourselves, our perceptions of others, our habits and so forth.. 

This is also the part of the mind that hypnosis, guided imagery and NLP are trying to reach.  So often times, when reading or discussing hypnosis the subconscious mind is brought into the conversation frequently.

Like any good bookstore the subconscious will allow any and all information to be stored here. And like any good bookstore, there are sections divided up into what I like to term reference materials, adventure stories, the old proven and time tested classics and the best sellers which are right at the front of the store for easy access.  Now unlike us, who takes hours to flip through reference materials, adventure stories and etc., the subconscious goes through all this information in speeds so fast it is impossible to measure.

The job of the subconscious is to constantly reference all these materials, so that we can go about our daily activities without having to spend an excessive amount of time thinking about how we may react, how we are going to move all of our muscles to get from one place to another, where we are going to get information to make decisions, and that sort of thing.  All this information is accessed constantly by the subconscious, which analyzes from its reference materials, compares that to how we have programed it to react in our best seller section, and completes the task for us instantly. 

The subconscious accesses this information to help us lead a productive and happy existence. It decides what makes us happy because we have told it what makes us happy, by repeatably placing the same information into the subconscious.  For instance if you find food to be comforting when you are feeling bored, your subconscious will automatically trigger thoughts of food any time you are feeling bored.  Because it wants you to be happy and content.

Now like any bookstore, information gets outdated and new reference material needs to be added so that we are actually drawing upon the best information possible to lead healthy happy productive lives.  However the subconscious mind does not place new and updated materials to how we might react, or how we habitually do something.  Even though reference material for instance tells us that smoking is not good for us, the subconscious is told how to react to that information by the conscious mind, and the actions that you do daily to reinforce how you want to react to reference materials.  It gives you what you want, the sole goal of the subconscious is to make you happy.

The conscious mind

The conscious mind is the part of the mind that we use in our normal, waking state as we go about our daily lives.  This part of the mind is very analytical, and deals with reason and logic.  This is the part of the mind that programs or directs the subconscious mind.  It also contains that little barrier I spoke of earlier, or what is commonly referred to as the critical factor. 

Before any information is stored or sent to the subconscious it must go through the conscious. The critical factor is like the doorway into the subconscious mind.  The conscious mind filters and asks questions about any new information that will or may be presented to the subconscious mind, the critical factor then looks at this information to decide if it will be allowed into the subconscious.  If the information is in conflict with what is already stored in the subconscious the critical factor tosses this information out as useless.  If the information is totally new and there is no reference to decide from the critical factor may or may not allow this information into the subconscious mind. 

It sounds a little complicated however it's really quite simple, once you have an understanding of the fundamentals of how it all works together.

Lets take the following example:

Alice decided as a teenager that she wanted to smoke.  She did this because she wanted to fit in with her peers and at the time it was the cool thing to do.  However over the  years medical discoveries have proven that smoking is dangerous to your health. So now she has decided that she wants to quit smoking to lead a healthier life style.  She makes this conscious decision, but finds that she is struggling in her attempt to quit. She keeps finding herself wanting a cigarette, the more she tries to quit the more she wants to smoke. She gets frustrated and can not understand why there is this defiance within herself. She soon however gives up on her attempt to quit and decides that she is suffering from addiction and needs help in order to achieve her goal of quitting smoking. 

This is where the critical factor stepped in and decided not to put this new information into the subconscious mind.  When she first took up her habit of smoking the mind and the body did everything possible to tell her that smoking was bad for her health.  By creating coughing, dizziness, and nausea when she first began inhaling that smoke from the cigarette.  She continued to smoke, in essence telling her mind and body to disregard the health issues.  So in essence this reason for wanting to quit smoking, health, was in conflict with the stored information of when she began smoking. The critical mind tossed it out. 

The critical factor is like a very stubborn bookstore keeper who is very fond of the books already in the store, even though they may be outdated.  It is set in its ways and follows the rule patterns that have been set to it over time. That is how the information in our subconscious mind becomes outdated, simply because the critical factor finds the update material to be in conflict with the stored material.

One could simply ask why we need that critical factor in the first place then?  Well the reason for that is quite simple too.  Although the conscious mind is very analytical and uses reason it is also easily swayed or distracted.  It tends to develop ideas quickly sometimes without reason. For instance one might decide this new diet on the market is the greatest simply because a friend of a friend lost 12 pounds while on this diet.  The critical factor protects these types of whimsical decisions from impacting the health and well being of you.

Hypnosis, Guided Imagery, NLP

Of course any change can be made, as long as the person wanting to make the change is persistent.  It takes repetitive action to retrain the critical factor to allow information into the subconscious. So just as when Alice ignored all the warnings from her mind and body when she began smoking, she would have to ignore all the resistance and continue to stand ground in changing.

This is where Hypnosis, Guided Imagery and NLP can help. These tools help by relaxing the body and the conscious mind.  Distracting it actually, so that the critical factor becomes less attentive and new ideas can be submitted into the subconscious without the critical factor blocking the entrance and preventing those new ideas from going through. By using  the tools of hypnosis, guided imagery and NLP the mind can be retrained at a much faster level.

 References

1. http://thetrainingdoctor.com:
2. www.uruniq.co.uk
3. www.gilboyneonline.com