What I heard quite recently is that you will get decision +/- 4 weeks after interview date. Admits will get the call, dings just email. If you are not hearing any news (positive or negative) within +/- 4 weeks your chances for waitlist significantly increase.
Based on the feedback so far it actually makes sense. Vast majority of decisions fits +/- 4 weeks timeframe and no waitlist yet.
But then again - just opinion of a friend of mine (albeit Kellogg grad who was helping with admissions) and I guess exceptions always apply.
EDIT: well... I don't recall any more whether the info I got was "after interview date" or "after your application went complete". But you get the point.