It depends on your goals for climbing and why you are doing it in the first place. When I started climbing I went all in and had 4 hour sessions on average I believe. I was quite obsessed. But, I also had a goal, to get as good as I could in the shortest amount of time.
Could I have done that with less time per session, definitely. I went 3 or 4 times a week, 4 hours each time, I wasn't really giving myself enough time to rest and recover, and I did get injured a few times. Looking back I would have definitely taken it easier and went the approach of quality over quantity. At least I would have had a better ratio to resting vs climbing. I would probably have been there for 3 hours at most and just 3 times a week.
But that is me and my body and my physical level, and that is how I can answer the question for myself, but for you, are you new to climbing? How long have you been climbing for? Would you say that you are generally physically active and are in good shape? These and more questions would help answer your question, you know yourself, and the key is to listen to yourself and to your body. If you start to hurt or get injured, reduce your climbing time. If you are capable of doing 3-4 hours at a time, go for it. If not, go for 2 hours even. If you can only do 1 hour, then do 1 hour.
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!
