Two hours feels too long. We've been doing independent playtime since my little one was about 5 months old, and now at 16 months, we do 30 mins 2x per day. I think 90 mins is about where I would max out. This site might help you:
https://www.babywisemom.com/independent-playtime-overview/
We found that the best way to inch the time up (we started at 10 mins) was to have an alarm that beeps. That way, there's no use arguing or complaining, you have to wait for the beep. I'd probably set it at 45 minutes for now, and once he gets use to it, start bumping it up 5 minutes at a time until you get to 90 minutes. You might also consider doing 45 minutes twice a day.