There's no timetable for a call-up yet. One thing I have heard is that the Rockies aren't terribly worried about Cust's weak glove. There are all sorts of possibilities here, including:
A trade, presumably to an AL team where Cust would serve as a dh.
A call-up, with Cust platooning or joining Pierre and Walker as everyday players (Note: when was the last time you saw the words "Walker" and "everyday" in the same sentence?).
Further seasoning in the minors.
Ok, that much is probably obvious, but the details are anything but. Here's what I feel is the most likely scenario (based on speculation, not inside information):
Cust may very well be called up in time for interleague play, where he can serve as a dh. From there, he should be eased into a reguler of role, although he could be there very quickly if he comes out of the gate with a hot bat. Sometime thereafter, possibly in the off-season, a trade seems likely, as I feel the Rockies see Choo Freeman, not Cust, as their regular outfielder of the future.
At any rate, he's an extremely intriguing prospect, and certainly worth a pick-up in deep mixed or NL-only leagues. If you like to gamble, you might also look at ways to pick him up in an AL-only league as well.