At any time, you can simply open your eyes and be done. Simply tell me that you don't think hypnosis is for you and I will count you out of it to make sure you are fully awake. That is your call and I won't be offended.
You can also say to yourself, out loud or in your head "One, Two, Three, Four, Five! Eyes Open! Wide Awake!" and you will end the session. I can do very little to keep you hypnotized if you don't want to be and I wouldn't anyway. If you are done, I will help you wake up.