I found out 3.5 weeks after I interviewed, but I think this is pretty abnormal. At the interview they said 6-8 weeks. I am pretty sure they meet monthly at the end of the month, so, at the end of December all of the November interviewees come up for review and letters are mailed. However, my interview was at the very end of the month, and they told us that it was unlikely out interviewers would get their material in on time to put us with the group from our month, and we would be rolled over into the next month. I was shocked when I found out hardly 4 weeks later because I was expecting to wait another month.
I actually felt like I did great. My faculty interviewer was a really nice guy and the interview went for an hour and 45 minutes (it's supposed to be 30min to an hour). We talked about all my EC's in depth, we talked about the area, we talked about, well a lot. I felt like we really connected. I thought my student interview was solid, not amazing fireworks, but solid. So, who knows what it was . . . maybe they just want to see if I really want to go there.
Edit: For reference my MCAT is well above their average, but my GPA is a little below.