Well, back in the old day, for our AD&D 1st ed campaigns, we used to have characters that got to 36 level or higher using variant rules that we made up. Ah, the glories of getting xp for gold and magic items back in the day.
In 3rd edition, the highest I've seen a party get is 14th level (not counting one-shots) and that was in a Scarred Lands campaign, although my FR campaign is at 12th level right now.