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.
Actually I think one of the biggest things about DX 11 is the improved multi threading support. Using DX11 can help games scale with more cores easier than developers having to code those optimizations manually. If you look at the developer build of windows 8 it's geared toward current hardware. I haven't messed with the DirectX template in it's Visual Studio much yet though.
DX11 is barely being used today. Why update to DX12?
DX10 offered little over DX9 and was quickly surpassed by DX11. DX11 will likely become the standard in years to come as Windows XP loss more and more of it's install base. Meaning more potential gamers will have a DX11 card and OS.
DX11 will most likely become the "new DX9". Meaning there will be a day when no more games (or at least the vast majority of games) will no longer support DX9, the basic common denominator for all games will be DX11. When will this happen? Maybe 4 years from now.
DX9 has been around since 2003, will DX11 be able to dominate for as long as DX9? Who knows? DX9 still dominates to this very day, but it is slowly fading away.
As I said the next-gen consoles (PS4, XBox) will be based on the very latest DX11 card and those game consoles should last for five to eight years. This will not encourage much development in DX12 since more and more games are designed for the PC and consoles simultaneously.
If I had to guess, I'd agree and say at LEAST four years for DX12. But again, what would it offer to make the switch, ray-tracing? Then they'd have to design PC-only gaming engines.
DX11 basically "fixed" most of the things missing from previous DX versions such as multi-threading and Tessellation so any game engine designed to fully utilize these features is going to be awesome.
Okay, I just have to drop this in there:
DX10 and Vista just to play Halo 2 (an outdated XBox 360 port). Lots of people realized that "newer DX" doesn't necessarily mean better. Then they had lots of performance issues with DX10 for little benefit. Then DX11 came out but they still haven't really utilized it because the percentage of DX11 cards is too low (minor benefit in same games).
However, DX11 when finally optimized will be AWESOME! (and if you understand how Tessellation works you'd know that it can draw only the quality you need and not buffer unneeded large textures so we'll get much better quality with less processing power)