In one sitting? 33 hours, Final Fantasy 7. One playthrough from start to finish and I cut a lot of corners.
Overall? According to Steam, Empire: Total War (159 hours), but that's only games I've owned ON STEAM played via their interface. For some reason Fallout 3 doesn't get its time counted on Steam (a damn shame really) but I'd say I've crossed the 200 hour mark with it.
But overall, all-time? I don't know exactly, but Oblivion (at least 800 hours), ESPN College Hoops 2K5 (730 games all played on manual, total of ~600 hours), Sid Meier's Alpha Centauri (hundreds of individual games), The Sims 2 (an entire neighborhood built from scratch on a savegame that lasted three and a half years until it got lost in a hard-drive purge due to a virus), a whole bunch of games that have approached or passed the thousand-hour mark.