In highschool and college, 8-10 hour game sessions were the norm. In highschool I ran my D&D game every Saturday, starting at 4 pm and ending when my parents threw everyone out (usually at midnight). The last weekend before college, we ran a little longer, but that was it. In college, we generally started after dinner, but, you know—college: we routinely ran until 4 or 5 am.
At some point, lives got busier, we got older, and sessions got shorter. So picking out a single longest session is hard. I’m sure it was some time in college, probably either the D&D game I ran freshman year, or the D&D game I played in the next couple years, but a memorably-longer session doesn’t come to mind. Sadly, I don’t have any of those marathon 24-hour or all-weekend game sessions to talk about. But despite that, it’s interesting to me that there’s just something about D&D that tends to lend itself to long sessions—our WoD and Ars Magica games in the same era with the same people were generally saner 6-hr or so sessions, and ditto for just about everything else I’ve ever played.