There are two things: the lifetime of a posthypnotic suggestion, and the lifetime of the hypnotic state. Posthypnotic suggestions can last at least as long as the subject's memory will permit. read more
Hypnosis can help cause a change; once the hypnosis ends, the change stays, but it can be undone if the subject behaves in ways contrary to the suggestion; or if rapport with the hypnotist is broken; or if the subject expects it will not last; or if the suggestion is forgotten. read more