Mass Effect 2 with about 18 hours (4:00 am to 10:00 pm).

Around 25 - 30 hours, done several times with different games, all of them on PC. Master of Orion 2: Battle at Antares (turtling on Impossible Difficulty), Panzer General 2 (full Guderians Strategy playthrough), Fantasy General (first 3 continents), The Dark Eye: Drakensang (until the end of Moorbrück), Supreme Ruler: 2020 (conquering half of europe with Luxembourg) and Heroes of Might & Magic IV (don't ask me how far I got, I forgot). Which one of them takes the cake though, I really don't know.

Can't say with certainty but an RPG that is really good can easily make me put down at least 8 hours straight.

24 hours Gran Turismo 4 with a cousin relaying me.

Did a few 12/14 hours marathons back in the day playing Counter Strike!
About 20 years ago, when I was in College, I did an epic marathon with one of my best friends who is really hardcore into gaming, we played Illusion of Gaia/Time from start to finish in one run: 32 hours, but we relayed each other & at some points I felt asleep or he did but it was really a great experience & a good RPG!!!

I had many long sessions during my childhood, especially with Pikmin 2; I remembered that I played from the evening to the next morning. Since then though, not many times, considering that school and work consume too much time.

i think it was 22 hours on FFXI when it first came out....non MMO would still be up there but not sure.

I dont know how some of ya'll do this those are some insane numbers

Usually any RPG, Minecraft, or Starbound. I could play those all day if not careful.

Even with a 4 pack of red bulls and a tin of chew I couldnt do past 13. Maybe with a bit of coke hahaha