I'm in a campaign that's been running since May of 1983, though I've only been in it since '88. Sessions have been fairly irregular for a while now, but close to weekly until around '90, when the DM got married.
A little less than two years of game time have passed (it once took a year and half to cover the events of one night), and we still have one of the original characters in the party, and a couple others around as quasi-NPCs. It helps that the DM divides all a the character's earned XP by their level, so that (under 1E rules) a cleric needs 2,250,000 XP to go from 10th to 11th lvl -- something the cleric in question hasn't managed to do in the 15 years I've been playing.
We should be converting it to 3E within the next few sessions.