Your body is highly efficient.  Often times, this leads to frustration as the body does not seem to respond to exercise and dieting as quickly as we would like it. In a perfect world, when dieting and exercising, you would see immediate results.  Unfortunately, this is not the case.  It usually takes the body six weeks to make any substantial changes.  The reason for this is that change requires a significant amount of energy from your body.  Being efficient, the body does not want to spend energy making changes unless it is confident that the changes are necessary.

For example, when you begin a weight lifting program you will not change the muscle (size) to any significant degree for about six weeks.  You will, however, see an increase in strength as the body will learn to perform the exercise tasks more efficiently.  Nerves will fire in a more coordinated fashion, your form will improve, and you will learn which muscles to best utilize for a particular activity.  But it is not for six weeks of continued activity before the muscle will be remodeled.  One pound of muscle burns 35 calories per day at rest.  The body is not about to put energy into creating new muscle that is going to burn extra calories unless it is certain that it is necessary.  After six weeks of continued demand, the body will consider that you're serious and make the necessary modifications to increase the strength and size of the muscle.

In regard to dieting, if you have a slow metabolism and you begin to engage in behavior that will increase your metabolism (such as exercising and eating more frequent, smaller meals) it will again take approximately 6 weeks to see any significant change.  In the past, your body may have considered itself to be in  starvation mode (this can occur from dieting; eating fewer than 1200 calories per day or from limiting yourself to eating only one to two meals per day). If the body thinks that food is in short supply, it is not about to speed up metabolic processes that will require more energy.  Again, it is only after about six weeks of engaging in proper activity (exercising and eating six small meals per day) that you'll begin to see a change in your metabolism. 

This is why you must be persistent with diets and exercise program that do not seem to produce immediate results. Sometimes there is need for additional modifications but often times it is simple patience that must be employed in order for results to follow.