When Super Mario Bros. 3 came out my brother and I played it for 12 hours straight (6pm to 6am). I've probably equaled or surpassed that with Mass Effect, Tales of Vesperia, Lost Odyssey, and Last Remnant though. Wasn't watching the clock so I can't be sure. If I get my hands on an RPG I enjoy and I play it on my day off. I'll pretty play the damn thing the entire day.

Not counting bathroom breaks(I wiz every hour or so)

My longest session was something from 24-30 hours. I used to be way into playing sleep deprived...

Now however, I find sleep more pleasing than video games. I'm too busy to play that much now.

Well over 24 hours, maybe an entire weekend with WC breaks, having small snacks at hand for not stopping for food. All of this in the Smash Bros Melee days.

In recent times, over 18 hours with Brawl.

Around 5-6 hours.

+ 12 hours on civilization, snacks at hand and maybe a bathroom break in between.
Same goes for FFVII, Command and Conquer and Ultimate soccer manager.

Probs around 2-3 hours, I have a very short attention span with games, they bore me quite quickly.

with no break whatsoever.... I guess about 12 hours, as a kid with Chrono Trigger.... started after dinner and stopped when I heard my parents move upstairs as they were getting up... didn't want to be grounded =D
Otherwise I don't recall an other time without any breaks ( I usually do 8 hours before I need to go on a break at work :p so I guess that's my average....)

I'm pretty sure I've played games for 16 to 20 or more hours in a row before.

10 hours on Gauntlet 64. That was so much fun. We were so far into the game, then it froze.

I played straight through Halo 3 without stopping, so that was probably around six hours. I've also had some pretty long gaming sessions with my buddies on Gears and Gears 2. Some of those lasted most of the day. When I was younger I played games for a few hours at a time easily, so I bet some of those would stretch even longer without me even noticing much.