Most hours spent on one game?

Garry's Mod, easily 7k+ hours on it last time I checked. If I were to guess, I think it was around the 7.3k or 7.4k mark. Still, easily gain like 10 to 20 hours a week. (Waiting for that S&Box, hopefully, it'll live up to the hype of Garry's Mod.)

Single day wise, I've spent at most I think it was 16 hours on COD MW2.