Back in high school my church youth group had a really silly fundraiser called a "rockathon". It was like a walkathon except instead of people pledging money per mile you walk they pledge money for number of hours you're willing to spend in a rocking chair. (We were allowed a 5-minute break every hour for things like using the facilities.) The one year I did it we "rocked" for 24 hours straight.
We started really early in the morning (like, 6AM, I think) and early on five of us started a game of Risk before the sun came up. Two people got bored with it and dropped out within the first hour (their territories became neutral) and I think we wiped the third person within the second hour, then me and the other remaining player continued to play until after the sun went down. The entire game was somewhere between 10-12 hours long.
We were both fans of what we called the "slug maneuver" where you mass up a whole lot of armies on one territory and use that mass to blast through your enemy's territory, leaving a single army behind on each territory in your wake (the slime trail), but we never seemed to be able to build up enough mass to completely take out the other person, and from the handful of territories left unconquered the opponent would turn in all of their risk cards to build up a similar "slug" and would blast you back. It was kinda like a slowmo ping-pong ball tournament.
The other really long game we had was Settlers of Catan. I'd often bring this came to Cape weekends (when a bunch of us got together in a cabin on Cape Cod for a weekend) but because there were only four players usually somebody had to team up or get left out, so I purchased the expansion that lets six people play. We started soon after we had brunch and before we knew it it was dark and we were all hungry for dinner, then I think we got back from dinner and played some more. That was at least an 8 hour game, but could've been closer to 10 hours. I was actually surprised everybody in our group was willing to sit still for that long!
We all like Settlers, but I'm not sure everyone'd be willing to spend an entire day playing it again. I wish the game could be shorter but still allow six people.
