I haven't heard any definite date mentioned yet. Zimmerman is just starting to throw again; we'll know more once we see how this goes. If he does return at full strength, I expect the Rangers to hand him the closer's role again, but even that isn't a given. If it were my decision to make, I'd probably leave well enough alone with Irabu and use Zimmerman as a setup man; the bullpen as a whole might be better off that way.
If you have him on your disabled list, be patient and hope for the best; if you're targeting him in a trade, don't give up too much for him until you've heard how his first throwing sessions went.