Daggerfall - god knows how many hours. At best estimate, ~800 hours. The sequels, I also played, but not quite as long as Daggerfall given I had lots of games (and less time to play) by the time Morrowind and Oblivion came along.
Mount and Blade (original) - over 400 hours.
Mount and Blade: Warband - over 200 hours.
Fallout 2: must have been over 300 hours - I never got tired of replaying it back then, making new characters.
Fallout 1: Probably got to 200 hours.
Eye of the Beholder 3: the first AD&D game I experienced. must have clocked at 200 as well, most of that wandering around the forest and mausoleum and trail endlessly, and experimenting with different party setups.
And way way way back: Steel Thunder, on a green-screen CRT on an XT processors powered PC with no harddisk - first "real" PC game I had. I can't count the hours I spent there. Including obsessing over every page of the manual that explained the tank, weapons, and ammo specs, ST could easily have gobbled up 400 hours of my childhood.