Probably about 35-40 hours... I went 36 hours when Dark Age of Camelot released. I think I did close to or just over 40 hours in WoW because I was close to becoming one of the first players the level cap back in vanilla. But I think only maybe ever went over 30 hours two or three times. I wouldn't make a habit of it, and I never used energy drinks or anything artificial to do so.
When I was doing the original HWL/R14 grind I played 20-30 hours lots and went a couple months even playing 15-25 hours a day, but soon as I'd get tired I'd always get a good 7 hours sleep. Sometimes guild mates would try regularly stay up for 30 hours plus but they'd always burn out or disappear for a day or two after lol. Sort of like the tortoise and the hair haha a slow and steady 20 hours awake//7 hour sleep wins the grind.
This was all a decade ago, probably haven't game more than 20-25 hours in a very long time... maybe something like... I think when Deus Ex Human Revolution release I must have played it close to over 20 hours straight -- I binged than hard in 1 weekend haha. But definitely nothing over 25 hours.