Change overnight would be nice but it's not realistic.  Studies have shown that it takes about 28 days to form a habit. Whether it be starting a new exercise program or changing your diet, you need to be diligent during those first four weeks.  The first month it will be easy to quit and fail.  Things will be hard but if you can make it over the 28 day hump things will get easier.  Once you have eclipsed this barrier and established good habits they will now be a part of your routine and much easier to maintain.

Four weeks is also an important number because that is approximately how long it takes for your body to change following implementation of a new program (diet or exercise).  The human body is incredibly efficient.  The body will not spend any extra energy that it does not deem absolutely necessary.  When you first start an exercise program, you cannot expect a change in your muscles for a least four to six weeks.  You may indeed get stronger because the body will become more efficient at exercise.  Just as when you learn to ride a bicycle, at first you are a bit shaky but as you practice your skill improves and so does your speed.  With exercise, your strength and endurance will improve as the body becomes more efficient at the nervous system level.  Your body will coordinate movement with greater efficiency.  But it is only after four weeks that your body will actually undergo change.  Changing the musculoskeletal system requires a tremendous amount of energy from the body.  The body, therefore, is not about to change at the first sign of exercise.  The body will wait to find out if it is truly necessary to make alerations.  After four weeks, the body will have decided that you are indeed serious and change is indeed necessary.  The body will then begin the remodeling process.

Dieting can act in the same manner.  If you have a had a terrible diet for years and then suddenly you begin to eat properly your body is not going to change overnight.  For example, many people only eat one to two meals a day. This slows your metabolism as the body thinks that it is starving a good part of the day.  If you then begin to engage in healthier dietary habits such as eating six times per day the body will recognize that it is now getting consistent amounts of food at regular intervals.  It will not, however, speed up the metabolism for several weeks as it will want to make sure that this new program is going to continue.  After four weeks have elapsed, the body will consider that food must now be consistently available and it will be safe to increase the metabolism. 

If you have been discouraged in the past by slow results during the early part of diet or exercise, this is why.  Stick to your plan.  If you're still not making progress after four weeks, this is the time that you should consider making additional changes.  Don't quit early.  Give your body a chance to change. Stay strong during those first four weeks. After the first month, it's all downhill.