We have never once gotten to 20th level in a campaign. I believe we got to 16 or so in 4th Edition, but that was in a more "DM mandates when you level" kind of game.
2E: One or two campaigns until roughly 10th-level, and one that started at 10th-level and finished around 16th-level.
3E: One campaign until 24th-level.
3.5E: One campaign until 20th-level, one campaign until 20th-level, one campaign until 14th- or 16th-level (I cannot remember which), and another until 10th-level before I broke down and decided to try...
4E: ... we've had a couple of Heroic Tier campaigns with another about to start. With 4E, we no longer aim to go higher than somewhere between 10th- and 12th-level. The story gets told and then we all get to try something new!