The topic of timing sessions has come up sometimes.
Something I'm about to try as a client is to set a minimum time (2 hours) and just see how we feel, and pay for whatever the actual time ends up being (to the nearest 1/4 hour I suppose). It seems like a fairly relaxing way to do things, and it fits both our schedules because we have nothing planned afterwards.
Have people ever tried that? I don't think I've heard it being mentioned before.