I doubt there will be a DX12 card before 2014.
One of the biggest things DX11 has going for it is Tessellation and that's going to take years to become a standard part of most games.
Game developers don't want to switch without a great reason. Tessellation is a really great reason and they'll explore that and newer cards will begin to offer greater tessellation ability to match games that use more of it.
Also, there is little doubt that the next generation of game consoles will use DX11 graphics. It's likely that will put us in a holding pattern with DX11 because game developers will be creating game engines to work across the PC, PS4 and "X-Box 720."
DX12 needs to offer something special to make the transition, like Ray-Tracing or something, but I think that's frankly many years away.
Also, don't forget that game developers always develop for the mainstream cards. Anything written specifically for the very latest cards is usually just a small amount of eye candy like the puddles in Dirt 2.
So never, ever worry about the next-gen cards unless some new cards are only a few months away.