Well it has been announced for both February and March, so I assume March. I think the last time frame for the Alpha was 4 months maybe two. I think the beta will come out in August, but somehow come out in April. If given my choice I would rather spend four months on the Alpha than two. Since it will give more flexibility on being able to change the final game.
For the most once the game comes out the expansions, Dlc, and mods will be somewhat limited on how far they can change from the original.