Speculation for 2013 but there are a lot of factors
The answer to this question depends when will Apple think that there's a ripe technology for providing a good retina experience on a 27" display.
The resolution of the display will be derived from what Apple think is the retina resolution for it. Apple have been diligently quadrupling resolutions these past 2 years, but there remains a simple fact that for over a decade graphic buffers have not been designed to handle more than 3840 x 2560 (yes a 4:3 resolution), and in recent years even less (2560 x 1600 even for the top of the line Radeon and GeForce). I assume there's a good chance we won't see a quadrupling of resolution for the retina 27", simply because it will not be necessary to achieve a retina feel. It might require just a 4k (4096 x 2304) resolution for that, which is a very achievable goal for modern graphics accelerators to push.
Resolutions per screens aside, it seems that modern graphics cards have to ability to push this many pixels but to more than one display. A rMBP was shown to run with 3 external displays, pushing a total of 14,860,800 pixels(!) (2560*1440 * 2 + 1920 * 1200 + 2880*1800). The total resolution for a real quadrupling of the current 27" Thunderbolt Display has slightly less pixels (5120 x 2880 = 14,745,600). But bare in mind that Apple would also not compromise on multiple displays, so you have to multiply this number by 2-4.
So I think that we might see a retina (4k) iMac by the end of this year, if they'll be able to push a top of the line graphics accelerator into it, that might be able to support up to one more retina display (or more, if they'll push more than one GPU inside it). This speculations stems from the fact that the iMacs are overdue for a refurbishment and Apple is still taking its time. Even if there will be such a device it might be sold along a refurbishment of the old iMacs. Similar to the rMBP and the new MBPs with the old design.
But I think there's also a very good chance that it will come by 2013, since Tim Cook was quoted to have something "very special" for professionals in 2013 when he was asked why the upgrade to the Mac Pro line was so modest.
I also think that Apple intends to retina all of its products, and they want to finish that move as soon as possible since, it's a lot of work, for seemingly small end-value (but a lot of glory), and they want to move to other features as well.