I ran a campaign in college from 1st to 16th. That's the longest that any game I've ever been in has gone. By the end, only two of the founding members of the party were still around.
The game I run now, and the game I play in, both show some signs of lasting a respectably long time - I'm angling, of course, for 20th level or higher. Mostly because I take great joy, as a DM, in watching the players develop their characters.
Of course, that assumes we don't strangle each other to death first...
Haven