In reply to Jackspratt:
I think it depends on what you are climbing and who you are climbing with.
If you are climbing on your own and pushing yourself then 20 problems in 90 minutes seems pretty good to me. Ignoring warm-up problems I sometimes only get a couple of problems done if I am pushing myself (today I only got one done in 3.5 hours but then did do over 30 easy problems to warm up and then flayed around on a bunch of others whilst also climbing with someone else).
I think it is better to think about what you are doing in each session rather than necessarily the length of time. Also worth mixing it up and potentially getting on whatever training boards are available, e.g. beastmaker. I find doing some long sessions of lots of easy stuff but really focusing on technique and moving slowly is really good for developing. But other times I will focus on very few problems that are really pushing me. At the opposite extreme sometimes I will do the easy circuits as quickly as possible and then can be racking up a problem every 30-60s.
Indoor bouldering is also, in my view, very sociable, so those spending longer than you might just be making more of the social time as much as the climbing.
I'm no expert but hope that helps.
