Either in terms of overall play time or just in one session.
The first time I played Oblivion, I was not aware I could lose track of 32 hours.
I've lived over an actual month in Skyrim, that includes eating, sleeping, and wanking
(That's .4% of my life)
[spoiler]#Gaming[/spoiler]
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I'm not too sure about one sitting, but I'm pretty sure Super Smash Bros. Melee is one of my most played games of all time... Dozens and dozens of hours of just pure fighting and stuff...