Depends on the program and where you are in their list. If you're a top choice, probably about a week or two after interviews. If you're waitlisted, you may be informed a week or two later or maybe not at all. If you're rejected outright after the interview, you usually hear about it within a month after the interview, but not always.
Basically, they go down the list. If you don't hear anything and you had an interview, it usually means you're still in the running because they don't want to outright reject you until they're sure they've filled their spots. One place I interviewed didn't reject me until July!
Most people ask at interviews when they'll hear back, or the DCT will tell you, from my experience.