Actually, in the states that I am familiar with, once you start you don't stop until relieved, OR until you are exhausted and can not continue. You can not just start, then after a few minutes decided that the victim is a gonner and stop for that reason alone. Having done CPR on more people than I care to remember, and for as long as 30-50 minutes, I can tell you that it is a very tiring process.

As far as watches go, I never removed mine, and never noticed a big problem with it, but that was just me. I am numb most of the time anyway...