Longest campaign played.
This is actually quite hard for me as the bulk of my game play in ye olden days wasn't really campaign based. Sure the players would keep their characters and move from scenario to scenario but I'd mash that up over various settings and use published scenarios from Dungeon magazine and various 1st Edition AD&D adventure supplements. It's worth saying though that we played a lot of 1st Edition AD&D over what was probably 5 years but it'd be stretching it to say that this was a campaign as that also included a foray into the world of Dragonlance for a good year or so.
So what was my longest actual campaign?
Clocking in at erm 2 and a bit years maybe with that being run pretty much fortnightly on Saturday's it'd have to be my D&D (very loosely) 3/3.5 game set in my homebrew / Dragonlance-hack of a world called Kilranthia.
I ran this at ORC Edinburgh and it was largely sandbox with some key plot hooks and NPCs as devices to see how the players moved the story on. The game re-used some earlier concepts and characters from previous homebrew games I'd ran including using former PC names as NPCs this time around.
Players were almost all teenagers for the majority of the campaign and there were some comings and goings but most of all I remember it for being just damn good fun to DM. Plus with the added bonus that we actually managed to finish with a fairly decisive conclusion to the campaign - the bad guys won...
It's a setting I've often been tempted to return to but the very nature of a sandbox game means it has to have the right group of players to re-create the same effect. Maybe someday though!