It's not that soon after DX11 which was released 2 years ago. Alot of sequels for software take about 1-3 years. Some of them take alot longer though.
I just installed my DX11 graphics card yesterday though because I just got enough money to afford it. So now I can play DX11 games. Although I haven't seen any games that actually require you to have DX11. All games seem to only require DX9 with DX10, DX10,1, DX11 only necessary for enhanced game features.
If DX12 releases with Windows 8, then it will be about 3 years since DX11. Then maybe they can release games and hardware that actually require DX12 instead of simply DX9 games with features that require DX12.