you hobby, you'll soon discover that every lady is different in how much time she is willing to spend with a client.
In most instances, the proverbial clock begins when you are face to face with each other (when you enter the room). However, keep in mind, that if you are running late (i.e. you show up 25 min. late), it is possible, you may not get the full time. She may have plans immediately after (they may or may have anything to do with the business) and can't accommodate your late arrival. Accept it and don't argue.
Now when the clock ends is completely discretionary. Its a matter of getting a feel for how the session is going and that takes experience on your part. For example, there are ladies who will begin to give little signs (starting to get dressed, or telling you about their plans for the evening, or asking you about your plans the rest of the day, etc.) perhaps 10-15 minutes prior to your end time. These ladies are not going to be generous with their time and pretty much expect you out the door as scheduled. In other cases, you may be lounging naked on the bed with the lady and enjoying the conversation and it could be 10-15 minutes past the session time and she has not made a single move to end the appointment. Enjoy the moment.
In general, I have found with most well reviewed ladies (and as a new hobbyist these are the ladies you should be seeing), 10-15 minutes over the end time is generally not frowned upon. Its mostly perceived as good business not to rush the hobbyist out. But again, you'll learn not to take advantage of her generosity. She is not there to entertain you past your session time.