In 3rd Edition, I ran characters in one campaign from 2nd level to 24th level over 3.5 years of constant play.
In 2nd Edition, I ran characters from 1st level to 20th, or in some cases 18th/18th. (I ran a variant on multiclassing where you had to accumulate the XP needed for both classes to level before you could advance, thus keeping levels equal. It was a friend's house rule that I enjoyed liberating for the campaign when it started up.) The game ran for 7 years, with the last four being rather sparse (only on holidays when I went back home, that kind of thing).
As a player, I've never had a GM with enough staying power to make it that far. As I get older, I am beginning to understand why...
With Regards,
Flynn