Never with any online games. I only got into Guild Wars for a few weeks and the longest session on that was probably 3 hours.
As for my max, on Windows I'd guess Baldurs Gate 2 or Elder Scrolls 3, maybe 6 hours straight. Take an hour break and repeat for an entire weekend.
But in truth, my NES and Snes days saw much more action.
I remember several times sitting down with a friend and playing Super Mario from World one to World 9, all the way through with no skips. We also had some awesome Final Fantasy 1 sessions. I can recall one that went from 8 Am to 8 Pm. We just ate and drank in the TV room, which was right next to a bathroom.
On Snes I remember beating Super Mario World in one sitting, not using shortcuts. And again, Final Fantasy 2 was a big time killer. The first time I played and didnt know what I was doing, it took me 24 hours to beat. Which I did in four 6-hour shifts over a weekend. (BTW, if you know exactly what you're doing and skip all the extra crap, the games takes about 8 1/2 hours to beat. 😉 )