I advanced a 1st level Fighter/Cleric of Tempus to 18th level over the course of about 7 months - A lightning fast progression, to be sure, but the DM wanted to play ONE game in which the characters made it to the "big time". It took us about 20 sessions of about 6 to 8 hours each, so about 140 hours of gaming.
My highest level character at a more normal pace was about 11th level, a Halfling Fighter/Rogue, at a period of about a year, about 40 gaming sessions of 6 to 8 hours length, so, about 275 hours gaming time, roughly.