Last night ended the 3.5 campaign I run, they were 15th (of 20). It ran 7 years. The campaign I ran before that started in 2000 when the 3.0 core books had not yet all been released, and ended end of 2005. It went to 16th (of 20). The several year Eberron game I was in ran to 12th of 30. (It had a closing one session at 21st, but that doesn't count.) The over 2 year 4e homebrew game I'm in we just hit 10th. The 4e dark sun game that's close to 2 years hit 8th, and while it sounds the DM is plannign for the long haul, one of the players hates paragon and beyond so I don't really expect it to get too much farther.
So, we seem to be hitting 10th in with some regularity in games that last over 2 years. In all of them the majority of the time spent in the campaigns have been under 10th though.
EDIT: Prior 3.x games I was in usually reached past 10th, btu that's because most of the group liked high level play. AD&D games usually went frrom 1st to OMG, hitting high teens or even over 20. But that was back in college time when we'd play once a week or play a whole weekend day often. That's a whole different time investment.