Thats 1,5y from release. 2-3y from general adoption. (Depending on MS ability to push it to Win7 and not only Win9+Win8.1)
Mantle is a bit better as that is 0,5y from release, 1-1,5y from general adoption or quicker, depending on paths for porting from XOne, PS4.
Metal same. Though general adoption may be accelerated by general level of support by Apple hw.
OGL is right now part way there. Just lack strong show (platform) case. So its hard to estimate general adoption. But it should start no letter then any other desktop API gain serious traction. (On the basis that "if we can make that faster, then not this?")
But APIs by them selfs are not so important. GPU abstraction concepts are more important. And here we can see 3 camps:
1) mobile/Apple, hw is just not so good to support 2) or 3) so CPU concepts are more crystallized then GPU ones.
2) DX/Mantle, explicit CPU parallelization, thread safe on CPU, assign jobs to sparate GPU parts independently on GPU
3) OGL lets remove API altogether on CPU, lets make GPU self sufficient on GPU side of things.
OGL abstraction system have better potential actually in my opinion for long term goals.
And most important are toolings:
Mantle will gain some tools from consoles even if PC specific wont come.
DX will get tools from MS. And that is main focus from MS.
Metal. Apple can do wonders here, will have to see if they are focused enough.
OGL. Valve is pushing this currently. Google may start to, as Metal hit Android hard.
Also ALL those APIs will need NEW toolings. Everybody start from clean slate.
Mantle, working driver, close coop with game devs.
DX, ??, close coop with game devs and gpu vendors.
Metal, working driver, ?? at least some input from some game devs.
OGL, working and proved drivers for AZDO, need more work for making specs for getting to long term goals, constant input from game devs.
Core (pun intended) of my post is that all APIs start from roughly same base line. (Nobody is really ready: Metal/Mantle do not have production ready drivers, OGL need to make its vision be heard loud and clear, DX is 1,5y off; Nobody have ready tools; etc.)
"War is far from won"
And of course OpenGL have its secret weapon. FLOSS drivers! GPUs drivers no longer are black boxes.