You will be able to wake yourself up at any point you choose.
Being guided into a trance state does not in any way imply that you are "handing your mind over" to someone else.
You are always in control. In fact hypnosis work is primarily concerned with teaching you how to have control over your own mind and create the life you choose!
If at any point in the trance you feel that you would like to wake yourself up, you can do so in the following way:
1. Tell yourself that you are going to count from 1 to 10, that with each number you will come more awake, that on the count of 8 your eyes are open and on the count of 10 you are fully wide awake with every normal feeling returned to every part of your body.
2. Count from 1-10.