Ignoring one-offs, the two significant campaigns I've run went 10 years and 12 years. The one I'm on now ought to be good for about that again provided there's people who want to play it.
For the few who run much longer games, I'm curious to hear how you pace it and how you keep it going that long. Nothing perjorative intended. I've just never been in a game that has run anywhere near that long and can only guess how you do it, especially for the over 10 year long ones
A few tips:
1. Slow down the level advancement.
2. Let players cycle characters in and out to mix things up a bit. Better yet, run multiple parties in the same world/campaign and let players and-or characters jump from one to the other now and then.
3. Accept the fact you'll have player turnover.
4. Slow down the level advancement.
5. Spend lots of time on world design, but don't tell the players all of it - let them explore the world slowly. (example: in my current campaign that's been going for coming up three years, my players finally saw the regional [i.e. continent-scale] map only two weeks ago; before that, all they saw were local maps. However, that continent-scale map they just saw was in fact the very first map I drew in the design phase; everything else was based off it)
6. Give the world enough history to be able to mine said history for stories when you need them. Go all the way back to the creation of the universe and birth of the gods if you have to. Come up with 6 or 8 key historical figures and determine how or if their influence is felt today.
7. Storyboard out a projected sequence of adventures and story arcs. It'll never work out like you storyboard it, but it'll give you an idea of whether you've got enough material lined up or not. Also keep a list of stand-alone adventures to one side, for when the players don't go where you expect.
8. Slow down the level advancement.
9. Give the players time to explore if they want to. Never rush from combat to combat without filling in some details about what they see in between and-or giving them choice about which way to go. (this depends somewhat on your group's preferences and whether you have to lead 'em around or not)
10. Right from square one, tell yourself you'll run this campaign till you drop.
11. Whatever edition or system you start with, stick with it all the way through. Suggestion here says go old-school; the newer editions just aren't as well suited for long campaigns and to make them work you'll have to put in a lot more effort.
12. Did I say to slow down the level advancement? Even if you do nothing else, doing this one thing will greatly extend your campaign.
Lanefan