Do you mean at one time or overall?
In one sitting: Resident Evil 5, beat the whole game on veteran and normal and half of professional by myself then half with a friend. Played 35 hours, discluding the fact that I cooked my self 3 meals and got a couple of drinks during loading screens/in between games. By the time I was done I was so tired I fell asleep as soon as I hit my bed, was asleep for almost 12 hours straight when I woke up I got right back on the horse and started finishing the mercinary modes.
Overall: Diable 2 LOD. Not a 100% sure, but Between ages 10-13 i played 4-5 hours a day, 7 days a week + around 6 hours on the weekends. Sooo... lets see, 365 days a year times 3 = 1095 days (days I wasn't playing are countered by the days that I got off from school) 1095 times [(4+4+5+5+4+6+6)/7] equals
5321.7 hours spent on Diablo 2 LoD WAHOOO!!!!!!!!!