This may just be me, we are all built differently, but I know i overestimate my performance capacity, and it depends on soooooo many things, the girl, my health, the amount of sleep i've had, what I ate the night before, it's hard to predict. Sitting in front of the computer all day I think I would looooooove to have a five hour session, only to find out after two hours, i'm beat, and I spend the rest of the night talking. I've also had a two hour session only to find out and hour later i'm horny again. And for some damn reason after a good session the night before I wake up with a raging hard on.
You idea is a good one, as is Miss Jezebels advice, most of the highly rated ladies only have one appointment a day so they may be free, but they do have other lives, baby sitters, dogs to walk, civie jobs, college classes, hair dressers, etc. So advance warning is good, plus it may motivate them to work on you differently so they can get the extended appointment.
In addition to you and miss Jezebels idea I offer the following;
If you do five hours make sure the lady is into that, five hours is a lot of work for them, if you check a few sites you will see many stipulate their five hour session requires 3 hours of public time, that means they may not want to "exercise" for five hours straight. I have had a few girls complain that sometimes things go on too long.
There are also some total nymphomaniacs out there that will leave you a spent shell of a man. They are harder to find, I haven't found any web sites that boast that capacity (although it sure would be helpful).
My recommendation would be to consider, based on your known performance capacity (and I am still learning mine, i've only been doing this less than two years, 16 girls, four trips to vegas);
back to back dates, one in the afternoon, one in the evening, or one in the evening and one in the morning.
Maybe a two girl two hour party with a followup the next morning.
maybe a date that starts with some fun, then a show or dinner break, and ends with a follow up.
if you're worried about not getting along with the girl and being stuck with someone you can't relate to (outside of the "exercising") go with your ATF.