The current longest forum i've been on is Prowrestling.com. Though nowadays I usually keep in the football subforum with about 10 or so other guys. Been there since I was around 14. Joined mid august 2004, so my 5th year there.
Also on a few Girls Aloud forums, though they are brutal when it comes to arguments. Load of young fans + bad spelling = bad times! This forum seems pretty calm when it comes to fans! so it's nice