Back in 2004-5, I ran a D&D campaign for my friends back in Michigan that only met once a month. To make up for having fewer sessions, we played long games: 8:00 AM to 4:00 PM, with some wiggle room on the back end to run over.
There was at least one session where we did run over, playing until 5:00 or so, and even setting aside our break for lunch, I’m pretty sure that’s the longest session I’ve ever run. So that’s my record: about eight hours.
Predictably, it ran long because we wanted to reach a good stopping point. We were all a bit more tired than usual, but it was a good experience overall. Although I’d run a game that way again if there were no other options, I much prefer more frequent shorter sessions — four to six hours seems to work best for me.
If you’re so inclined, I hope you’ll share a little bit of backstory on your longest session. I don’t think of this question as a pissing contest — I’m more interested in the details: Why did you run such a long session? Was it fun? Would you do it again?