I normally don't spend more than 2 hours at a stretch.
The two times I remember distinctly were:
1)
The Sims on PS2. I bought it the night before a huge snowstorm, played it for probably 6-7 hours the next day. I remember thinking about the irony that I was making sure my Sim ate, took a shower and used the bathroom and neglected doing all those things myself.
I sold it back the next day.
2)
I played Ocarina of Time for 8 hours straight while my wife was out one day. I was so miserable at the end of the day, and felt like I wasted my entire day.
However, now that I have maybe 3-4 hours a week to play games, I'm craving the possibility of an entire Saturday to play games.