I used to never go above 11th level. The current campaign I am running however is at 13-15th and still going. We have sworn to reach Epic Level but time will see.
Paizo, it seems (since there "campaigns" are all designed to be run over 20 levels).
I think it's interesting that my other poll implies that most campaigns don't come to a conclusion, yet that the most popular answer here is that about 2/3 of campaigns last from 7-15 levels. I admit, that's suprising to me.
Not counting one-offs, I've only ended one campaign; that in the 8-10 range. My current campaign is nearing its end, and will probably be in the 8-12 range at that point.
These are 1e levels, mind.
Edit: I should read the damn subject line...you asked about 3.x campaigns, which I have never run...so please ignore one vote for 7-10.
By 'your' I'll assume you mean what the I'm the DM of and not one I'm simply a player in. In which case the 16-20 level range is appropriate. I tend to create a stable game with a stable group of players and characters (though ofc both players and characters aren't the same as when the game began).