I know I played Arena, and later Daggerfall, for a long, long time each. It was across multiple saves though. I couldn't really say how much time each save got.
Both of those games had land masses much larger than morrowind or oblivion, which means more cities and dungeons to visit. But, on the downside, much of that content was randomly generated, using set parameters, so you wouldn't exactly be seeing brand new content for everything you'd explore.
You know, I haven't checked, but if they have those games up on gog.com, I might just go buy them, and play again. It would at least be easier than trying to get my original discs to run on windows 7 on my own
edit - The only game I know I broke that many hours playing is probably the original Everquest. I spent a good 5-7 hours every weekday, and much longer on the weekends, for 6 or 7 years.