Best guess was right before the Alpha was released but, then again they could use the Alpha release to encourage the Founder's Elite package so may be available until Beta or after.
My guess is Stardock has an idea of when they are going to cut it off but it's still flexible in there mind. My guess would be whatever gives them the best finacial edge.:)
Based on all the information present this is what I would do if I was making those decissions...
Founders Elite package would be available until Beta to encourage people to get Alpha access and pay $99.
Founders package would be available until a month prior to release to promote founders package.
Offer deluxe edition thereafter to include select expansions/dlc at $79.99
Ultimate edition to include all (same as founders elite w/o the founders part) $99.99
Retail version at target cost of $49.99