The highest level we've ever reached was a 12-year campaign (more or less) than I DMed through high school and college and for some years afterward. PCs reached around 22nd level in 3e terms (the wizards were 21st and 22nd level respectively, the rogue at 30th level *and* dual-classed!). Loads of fun; the campaign morphed from a local-intrigue campaign to a warfare campaign to a plane-traveling game to a world-saving game to a dominion-running game. Good stuff.
I've run a B/E/C/M/I campaign that took PCs from 1st to 36th level and thence to immortality, but advancement works differently in that system (IIRC, 36th level equates to 18th level in 2e terms according to the Rules Cyclopedia).