This is an oversimplified explanation of how long we should continue to use a hypnosis program:
We subject ourselves to pressures and stress every single day. Stress is the underlying cause for most of the symptoms that people want to get rid of like: Excessive eating; smoking; nail-biting; facial tics; Insomnia; etc..
The essence of hypnosis is relaxation. So when we listen to a hypnosis CD, that helps to drain off that stress and thereby reduce the unwanted symptoms. Of course, at the same time that we are relaxing in a hypnotic state, the hypnotist is presenting post-hypnotic suggestions to the unconscious / subconscious mind which also reduce or eliminate the unwanted symptoms .
So, we can either stop subjecting ourselves to pressure and stress, or we can practice self-hypnosis at least once per day, for as long as we need to. It is almost that simple.