2 hours at Pompeii was enough for my wife and I. I am a history buff and if I had been by myself I might have wandered for several more hours but I know the wife couldn't handle that. What we did see satisfied me and I got to take a ton of pictures.
So I guess the answer depends on your level of interest. If you are a history teacher you might want to take tons of notes and draw some maps or something, which would take more time. But we didn't feel rushed at all and I'm guessing we were there about 2 hours.