I can't recall what the longest single playing session was - there are always breaks for pizza & bathroom, discussion, or even partying. In ye olde days of high school we used to play during the day and party during the night, sleep over at a friend's place, and continue playing the next day.
As for campaign play, we have a massive middle-earth campaign going for ~14 years now. Three main GMs taking turns, each of them with his own master storyline (though they're intertwined), some standalone adventures, and anywhere from 2-8 characters available to each player, sometimes switching characters during an ongoing adventure or playing two or even three at once if the story allows for it. Though we found playing multiple characters at the same time doesn't work very well, and only do this these days if circumstances in the ongoing story demand it.
Needless to say, over 14 years the story has branched off a bit. We've concluded some parts, but the after action reports cover so many characters and areas that it starts reading a bit like the Wheel of Time or Song of Ice and Fire.
We've also created two spinoff campaigns, one switching sides and playing as dunlending/hill man auxiliary forces of Angmar, and one in a more arabian-nights'esque setting in and around Umbar (which doesn't take place in any specific time and may or may not be connected to the main campaign).
