My guess is that these will be around until the 4th quarter of next year, unless there are fairly major issues with the 1st gen.
They'll want to run the current (new) model for as long as they can so component prices come down and there's more margin in them. I'm guessing they'll be paying a bit of a premium for some of the GPU's in the new higher spec models, and it will take a while for these prices to come down.
They're also not being released for another month or two, AND the Mac team will be working hard on the new Mac Pro redesign that is going to be released next year.
Q4 next year at the very best. Haswell desktop CPUs are not due until summer, so with intel I bet September before they ship in volume, presume apple get a few to test with in Q3, I would guess an October release.
Haswell, Option of "Retina" I think we will see if its going to happen early Q1 or Q2 if apple offer it on an updated Cinema Display. Latest GPU. Depending on thermals, they may reduce the bulge.
Other than that I'm not expecting anything other than a silent refresh, 2013 is going to be the year of the Pro and seeing if Apple can keep ahead of pack with iToys.