We're closing in on 11 years. That was never the original intent; we just kept going because it was (and is) fun. Someone asked me recently how to make a campaign last for longer than normal. Here's what I think....
1. Stable players. It's almost impossible to do until everyone has settled down into regular jobs and out of college.
2. Changing up the adventure style. Too much of one type of adventure, and everyone gets bored. That's the problem with the middle of Monte's RttToEE and Bruce Cordell's RttToH; not enough variety, not enough fun side quests. Some horror, some high adventure, some political, some dungeon, some wilderness, some city. Include some adventures where the PCs gloriously kick ass, and some where they get thwarted. Make the plot move fairly quickly, glossing over boring parts and concentrating on exciting occurrances. Who really needs to know what happens every minute of every day or travel, anyways?
3. Long term goals. Set up long-term plot arcs, like the current White Kingdom adventure in my game right now, or Sagiro's over-arcing plot line (he's the best at this I've ever seen.) Intersperse them with short-term arcs, repeating enemies, and adventures based on PC plot hooks.
4. MOST IMPORTANT: Solicit player feedback to make sure that the game is mechanically healthy. If someone isn't being paid enough attention to, or they think combat is too slow, the campaign will never last. Get your ducks in a row, and then worry about plot.
5. Don't railroad or script adventures ahead of time. Embrace the chaos caused by smart players. Change the world based on what they do; statues of them might be put up, your long-term villian defeated WAY early and his sniveling underling takes his place, and so forth. Don't be afraid to destroy "sacred cows" if it will make an impression; got a boring city? Wipe it out with plague, or have an army invade. This sort of thing makes the world come alive, and if the players can see the impression that their actions make in it, they'll want to keep playing. A lot of times, public recognition is better than any monetary treasure.
6. Slow XP advancement. We go up levels once every 12 sessions, or twice a year. Decide how long you want the campaign to last and see how often you play; then prorate xp accordingly. Play every week, and want a 5 year campaign? That's about 250 sessions over 20 levels, so the PCs should level roughly once every 10-12 levels. Play every other week, and want a 5 year campaign? PCs level every 5-6 sessions. You should adjust XP gain based on this, dividing standard xp by 5 or 10 or whatever to reach the correct numbers. Make sure the players understand that it will be a slow advancement game.
7. Involve the PCs in the world. Build upon their plot hooks, surprise them, make their actions change the world around them. Once the players learn that their actions affect the world for both good and bad, they'll want to keep at it.
8. Only play with people who you'd want to hang out with outside of the game. If you don't like a person, you aren't going to want to game with them long-term.