Actually, this is not possible,
Once in this altered state of mind, if the therapist left the room, you would eventually come out of the trance state, usually in minutes. This is simply because it is something you do on a daily basis anyway. So, although you may not realize it now; you are most likely very proficient in accessing and using hypnosis. The only difference, in the clinical environment, is that sometimes apprehension, anxiety and doubt can interfere with the client's ability to effectively access this altered state of mind. Again, the experienced hypnotist will recognize this and find subtle but effective ways to assist the client.
Some people may see this as a form of manipulation but it isn't really! After all, why would you go to see a hypnotherapist if you did not want or expect to be hypnotized? In this sense, the hypnotist is merely facilitating your intended purpose in being there? It is also worth noting, that when induced by a professional hypnotherapist, the hypnotic trance can often produce a very profound state of deep relaxation; perhaps deeper than you have ever experienced before? Consequently, it is very common for clients to say "I didn't want to come back!" The way I allow the client to regain their sense of normal awareness is in their own time; naturally!