Well, that depends on how you define hypnosis. If you think along the line of our consciousnesss level, then hypnosis could be forever.
Hypnosis can occur at Alpha, Theta or Delta level. When a person falls into coma, he most probably entered into Theta or Delta level. He could still hear if you speak to him.
If you define hypnosis as a therapeutic approach, then it cannot be forever. Even if you were to fall asleep during hypnosis, you will still wake up again.