Usually after the interview they have some scheduled day for the admissions committee to meet up, look at everything and decide whether you get in or not. At one school I Interviewed on Friday and I knew the decision after the weekend on Monday, while for others I had to wait a month. In the end it just all really depends on the school, but I don't think its ever much longer than a month since a lot of adcoms meet monthly if what I was told is correct.