(Not a HDH, so forgive me for intruding--but I do tend to like reading what you guys say.)
Heinz, I agreed with you until you said "several hours between clients."
I mean... for instance, lunchtime is my most popular offering. (I'm not always available--a few days at a time--so availability is a thing.) Lunchtime also lasts from about 11 to 2, let's say. (Going by restaurant time.)
If I have three people who want to see me, but only have about this gap to work with, then... either I can see one of them (and tell the other two to pound sand), and have a couple of hours available if he wants to extend... or... I can put half an hour between (fifteen to freshen up, fifteen wiggle room for early/late), schedule a 10:30-11:30, 12:00-1:00, and 1:30-2:30, and make everyone happy while giving them what they asked for.
Of course, this only really applies during the high-demand periods. (I've got certain folks I know will go over and I schedule extra time for because of that, and thankfully none of them are in that spot.) But... why would it make more sense to hold out for the possibility of excess time (generally at a discount), rather than a "sure thing" of seeing multiple people who want to see me?
Particularly because... I've never had anyone try to upgrade, so it seems a little bit like planning for an incredibly unlikely eventuality.
I say this mostly because I started by making sure to put a lot of time between clients. It mostly results in a lot of unnecessary twiddling of my fingers and telling interested folks I don't have time for them.
If you frequently run into the issue, though... I did have something that worked, once. A guy wanted an overnight, but we'd never met, so he wasn't sure if we were going to "click" or not. So I offered him an option--a slightly expensive hour, and if it goes well, the overage goes toward the overnight price. If you'd like more time but aren't sure you'll use it, maybe that's the sort of thing to look for.
Again, sorry to intrude. But several hours seems.... excessive.