CEO John Legere stated at the T-Mobile's "UnCarrier" conference that the device will arrive "about May 1st," which means, providing no major hitches arise, consumers will only have to wait around five weeks to get their hands on Samsung’s latest and greatest smartphone effort.
Considering that the other carriers want to be able to release these phones in and around the same time so as to provide their competition no advantages in the market, I see AT&T, Sprint, and Verizon to be releasing the Galaxy S4 around that same time, plus or minus a day or so.