Longest i've ever played was back in 2006, SH3. Started after work on Friday, and played until i had to go work on Monday morning. Took two to three hour naps on the couch, with the pc volume turned up, but that didn't really work out too well.
I do remember by mid-day Saturday, i gave in and turned the tv on while i was still playing.
All in all, it wasn't as much fun as i had thought it was going to be. Went to work on Monday, exhausted.