Morrowind - Only like 10 hours or so, never really could get the hang of it.
Oblivion - Maybe 50, I dunno. Across many characters, I had a tendency to get bored. I rather like Oblivion but I keep on getting distracted by other games that came out recently.
Since it's a Bethesda game I guess it's related, I've spent way too much time on Fallout 3. Maybe about 150+ hours across many playthroughs? I rather hate the main plotline of that game, but exploring was fun. Too bad hitting the level cap in that game was so quick, and by then you maxed out like every skill. That kinda killed some of the fun at the endgame.