2hrs is the minimum time to digest a normal meal, I try to allow 3 to be safe. If your dragon eats above normal meals ( some do ), more time is needed. You can't be to safe!
I'd like to allow more time but his lights on/off schedule is based on me going to work and getting home. I won't have time to feed him in the morning if I push the lights off time back. He usually gets 2 1/2- 3 but sometimes it's 2
My mob have been on 16 hrs since little, so what if they have a nap mid afternoon , or put themselves to bed before lights out, other nights they are still up (because they are interested in what's on TV or what's going on in (with the humans) or want snuggles before going to bed.