1-18 Age of Worms currently. Signs are that we may abandon the campaign, since we're still a good half dozen sessions from completion and there are several good reasons to stop it and restart. We'll see tonight what everyone thinks. That's probably my longest run, and even then there's only one player who's stayed with the same PC the whole time. Some have died; I abandoned my original concept at 10th level when it just wasn't working out with the campaign, and did something else.
Honestly, I'm not that fond of the 1-20 paradigm. I prefer a 3-12 or so. I particularly don't like high level under the 3rd edition ruleset. I know one of the design goals of 4e was to make higher level much more playable, but honestly I'm skeptical that it's going to work out and not be a mess. I think the current system categorically is a mess once you hit the mid-teens or so and beyond. For a variety of reasons.