While it is difficult to be sure of the truth I suggest the following. Some girls are able and do take back to back bookings hour after hour. This is, after all, the business model in the walk up. However most WG's are neither this popular nor physically able to sustain this, so while they may do back to back on occasion I would think the number of girls who spend 8 hours of an 8 hour shift fucking punters is relatively low.
What I see much more of in London is 2 or three 3 girls sharing one or two rooms and occupancy (room use) running as close to 100% during working hours as the manager (pimp) can organise. That girl tapping on the door of you room when there is five minutes to go wants the room for the next WG and punter - it may be the WG you just had (if she is lucky and busy), it may not. But the real estate is expensive and the overhead has to be paid so they keep the rooms in use as much as possible during working hours. If you know where to look this also creates a downward pressure of fees, because the manager of this kind of outfit is more interested in busy girls than girls who can command high rates.
Understand this and it will sharpen your confirmation process. The worst outcome from a booking for a girl is that you book a session, show up, but then refuse the session and leave without paying. She has lost the fee and the room will be unoccupied for as long as you book. Many girls will try to avoid discussion of service and fees to secure a booking, but they would rather refuse your booking than confirm fee or service to have you walk without paying because the story changes.
My final text for confirmation routinely reads
"Confirming 1.00 pm 1 hour French kissing, oral without, deep throat, body to body massage (with oil), penetration, cum in mouth, £110. Please confirm."
No reply at this stage or - even more commonly - last minute withdrawal of service or admission of extras is driven by the fact that the WG does not want to lose your booking, but would prefer this to than you show up, refuse the session because of misrepresented fees or services and leave her with the cost an empty room for an hour.