1) Not before 10/15 (or 10/17 this year)
2) as quick as 5 days if they like you (my student host heard back in 5 days)
3) as long as till end of the cycle if they think you're so so
4) as quick as 5 days if they don't like you at all
I interviewed at Michigan on 10/1 and was notified 10/15. That was the fastest notice I had. My longest was about 10 weeks (interview 9/28, found out ~12/15). It'll vary widely, though.