I'm not a clock watcher. I do set a discreet alarm for the end of the booking, but if the fella is on the brink of cumming, I won't stop until he has finished. Sometimes I have gone over by quite a bit, maybe twenty minutes without realising. The chap has paid sometimes, but I don't insist on it. I know some guys like a chat to break the ice first or just after to get dressed so that twenty minutes compensates for that. It doesn't happen often.
One guy recently booked half an hour, and it was very hot weather. We were done within twenty odd minutes so I gave him a tenner back.