Leagues are signing so I would imagine it will require more time to give proper notice to get out of your conference. That's not the case for BYU obviously, but we still need 18 months at least to restructure our game contracts.
I would guess if the big12 is going to expand for 2017, we'll know by March 2016. That gives everyone enough time to get their ducks in a row while also giving the b12 time to see if they get left out of the CFP again.
Unrest from conference members, like Oklahoma, could push the time table up but I doubt it.