Averages are tough to approximate without hard data and silly me I never kept my time cards or pay stubs from those old games.
I played in a 1E campaign that lasted nearly 10 years into the 2E era before it finally folded. About that time I started DMing and my campaigns lasted 2 years at best but a number of them folded within a year when summer came around and none of my players felt like spending weekends or evenings playing D&D instead of dating, going to the beach, etc. I never could understand that.
3E I had a campaign that ran for a solid year and a half and would likely still be running but I had commenced it with the unshakable intent that it be a game of finite duration, ending with the story arc and before the PC's exceeded 20th level. I liked having COMPLETED a "game" of D&D at the time but I haven't had a proper campaign since. They've been extremely short as I can't get my players to commit to the game anymore which is directly attributable to MMORPG's, and to a lesser extent other computer games. Asheron's Call was a problem when all my players were into that (thank God none of them played/liked Everquest!), but WoW has been a complete D&D killer. That combined with losing all my old players because we've all moved has now put me at square 1.