How Long is an Hour (or Half Hour)? .. AGENCY INPUT WOULD BE APPRECIATED

Okay, sorry if this is a redundant question but I haven't seen threads addressing this issue on the forum. How long is an "hour" or "half-hour" in respects to a session (incall or outcall)? Does it start when the girl knocks on the door or does it start after she has been "freshening up" in the bathroom for 15 minutes? If a lady wants to take a 15 minute shower afterwards is that also included in the "hour"? Agency clarification on this issue would be appreciated; other input is also welcomed.