Yeah, I think a lot depends on the MLB staff. One factor for the earliest possible call-up might be a successful and clicking 4-man rotation (presumably Burnett, Wandy, J-Mac, and Liriano, with Morton still injured), which could allow Cole to be eased in. He's struggled initially when he moves up, so it will be nice if he doesn't have to step in to a situation where there will be a lot of pressure right away.
In short, though, I think the big factor is how well Cole is pitching vs. when there is availability/need in Pittsburgh. Smart money is on mid-season. I think if he has a solid first two months in AAA, he will be here at the very first opportunity.