I think what the OP is really asking is, if a client has only booked 15 minutes and takes a shower, does the shower time come out of the 15 minutes? The best way to resolve it is to ask the SP before booking. She should not count shower time against the session, regardless of its length, though.
I would never book a 15 minute session, but my answer would be that the client is always expected to be fresh and clean, as is the SP. If the booking is very short, then, at the very least, he should shower immediately before leaving for the appointment, and qiuckly wash his ass and "accessories" again when he arrives.
And, if she counts shower time against the session time, or doesn't care if the client washes, book someone else.