Hmm... I picked 12-24 months, but going over the tabletop campaigns I've been in...
(6 weeks) Sep 2007 Star Wars Saga game (goes on hiatus after next week; will be back)
(19 months+) Dec 2005 - Aug 2007; D&D 3.5 Eberron game (so far; on hiatus, resumes in two weeks)
(10 months) Feb 2005 - Nov 2005; D&D 3.5e Eberron game
(8 months) Jun 2004 - Jan 2005 D&D 3e/XPH Dark Sun game
(10 monts) Aug 2003 - May 2004 d20 Modern/Future game
(11 months) May 2002 - Apr 2003 homebrew D&D 3e game
(~9 months) Fall 2001 - May 2002 homebrew D&D 3e game (I joined the group after the game started, and don't remember exactly when I did)
(~2 months) Summer 1998 Earthdawn game
(~3 years) Fall 1995 - Spring 1998 homebrew D&D 2e game at college (alternate weekends while classes were in session)
(~4 years) Fall 1994 - Spring 1998 homebrew D&D 2e game at college (alternate weekends while classes were in session)
(~3 months) Summer 1994 homebrew D&D 2e game run by my sister's boyfriend
It's probably worth nothing that most of the campaigns I've played with my current group have had more 'table' time, being played pretty much every week year-round (except when interrupted by holidays, vacations, business trips, and family emergencies), than the games I played in while I was an undergrad, which were pretty much ever other week while classes were in session.