I would imagine it also varies from girl to girl. If you are seeking a girl in high demand, the agent would likely be looking to maximize his profits by keeping the girl fully booked. If you select a girl that is not meeting her daily quota, you may have some negotiating power with the agent for an all day session.
I must admit, I have never had the inclination or opportunity to pose this question to an agent, so it is simply idle speculation on my part.