In my experience with agency girls, the girl texts the agency when you arrive, and has to text them again when you leave, so the time between those two texts is what you get. The same applies with most independents, you are paying for the time you spend in her apartment. Very common to be asked if you want a shower toward the end, so the girl knows to usher you into the shower with 5 minutes or so left on the clock.
Personally, I feel that if a girl insists that you have a shower first, that should not be included in the time. If staying locally, I tend to have a shower at my place/hotel before I leave, so any starting shower at the girl's place can be quick. Although TBH, the more upmarket girls who insist on you having a shower first tend not to include it in the time, and are usually more relaxed about timings anyway.
I tend to avoid the kind of girls who do back-back bookings, not least because their hygiene is suspect, but also because they tend to short-change you timewise. Not uncommon with these kind of operations for your time to start when she sends the final text directing you in.
I learnt early on in my punting career that it is not a good idea to arrive any more than 5 minutes early unless you have texted to confirm that is okay. And as a previous poster noted, you certainly shouldn't expect extra time if you do.