I got mine two days after the callback interview. I think the general rule of thumb is anywhere from right after the interview ends (rare, but done by some firms) to about one week or so. If you have not heard by then, you can follow up with the recruiting office to reiterate your interest. Who knows, they may still have you on the shelf until they see what some of your competition is like. If you have not heard after two weeks, you probably got dinged.