Its not about time in the game world. Its all about the actual time spent on scrub encounters in the real world. A fight we had with a group of kobolds outside near a waterfall took 15 rounds to finish. Not much time in the game at all-but after an hour and a half grinding on a non-climactic battle our eyes had glazed over and we (the players) were mentally fatigued by the whole thing.
If that's the encounter I'm thinking of, it's pretty atypical...