I have spoken to some ladies who like and some others who dislike multi-hour appointments. I don't think the ladies make many mistakes in their pricing; I think it generally reflects their preferences.
For most of the ladies I see (selection bias is at work, this is not necessarily true of ladies as a whole), they prefer longer appointments with dinner dates a plus. Without the overhead time for clean up and rest between appointments, it works out well for them financially, particularly if they are low volume. Also, it may be less of an emotional drain to see one person for a longer time than two for shorter times. And hey, they have to eat anyway too. Also, unless you are trying to be a commercial for ED drugs, they will spend less time at it in bed and more time talking or in other activities that they may find more agreeable.
On the other hand, that means that they will have to spend more time with the you the client, and if they find that a chore, they may prefer to have less rather than more time. Also, regardless of how they feel about you or other clients, for some ladies it is draining to have to be "on" for a longer continuous time period. Even if she is an all-time-favorite for you, she will generally be focused on pleasing you rather than being herself during your time together (which is what you want, right?). How long they like or feel able to maintain this varies between ladies.
In my limited experience, ladies who tend to become friends of their regular clients charge a lower per-hour rate for long or overnight appointments and they tend to be more selective to keep the kinds of clients they like. Those who don't tend to make friends with clients have a more of a flat hourly rate structure or even one like Paloma's that discourage you from sticking around.