That could have changed now but I've never seen any official info to support that.

AAC as others have said is a 27 month notification if they are to depart without financial penalty. Anything less will have a large penalty (the shorter the notice the higher the penalty) what ever penalty is sought by the AAC will be arbitrated for a final amount both sides agree upon (likely less than what is asked for). Note that I believe Louisville paid $11M to the Big East/AAC. They announced toward the end of November 2012 and they started in the ACC on July 1, 2014 which was just barely over 19 months from announcement to official start.

http://espn.go.com/college-sports/story/_/id/8685541/acc-votes-add-louisville-cardinals
http://espn.go.com/college-football/story/_/id/11161888/louisville-cardinals-official-move-acc-big-step-conference

BYU does not have a deadline for football. WCC might for other sports but I'm not aware or worried about what that would be...