how long should each climbing session be?

I would add that it also depends on if it's indoor climbing or outdoor climbing. I don't climb indoors as much as I do outdoors, but usually when we go out we easily have full day sessions, leaving in the morning and getting back at night, sometimes the next morning even.

I doubt I would be able to have such long sessions in an indoor gym though. There is something about the outdoors that just makes it possible to have such long sessions. I think that for indoors my longest session has been about 3 hours.

Hope this helps!