For me, a Lot!
RO : March 2008-current
HPF : Aug. 2007-current (Way less active though)
SNF : Oct. 2006-Nov. 2008 BANNED
E2000 : Sept. 2008-May 2009 Stopped posting
AF : June 2009-Sept. 2009 Left ; Feb. 2010-May 2010 BANNED
For. Forums : Dec. 2008- Jan 2009 Left
Lots of Pokemon forums : 2006-2009
R. Reagan Forums : 2010-2010 Left
Some other right-wing forum : 2010-2010 Left
Freedom Forums : Jan. 2008-April 2008 (My forum, I deactivated it, when I was forced to resign via mutiny.)
PokePlace : May 2007-Dec 2007 * Was mod and Admin ; place was hacked and destroyed.
StarWarsForums : 2008-2009 Left.
I have two Interwebz homez RO, and HPF.
So, I guess you can call me a former Forum joiner, when it comes to forums.