Just depends. Your interviewer has to send the interview info. to the school. This can be delayed for several reasons. Such as: 1) waiting to do another interview and submit results of several interviews at one time. 2) waiting until they get some time off from work 3) procrastination (doubtful)
After the results of the interview land on the Admissions Committee table, it still has to wait for the AC to meet again. If it hits the table one day before the meeting, it gets reviewed quicker. If it hits the table a day after they met, it waits and gets reviewed at the next meeting.
That being said. I got accepted in 3 weeks. My old roommate with a better application, was accepted a little after 8 weeks had passed. Although, I applied in Jun04 for a Jan05 slot and my roommate didn't apply until late Sept 04. There were probably less app.'s to go through in late Jun.