I haven't read all of the other replies, but I guess it is probably obvious by now that there is much variance in this area. I just wanted to bounce in and say that nothing is certain- my period returned at a mere SEVEN WEEKS postpartum, in spite of frequent and exclusive nursing!
ETA: Now I have read the other supplies, and I don't feel so weird! Seriously, when I tell people how early my period came back I get one of two responses- people either don't believe that my information is accurate or they think that I am fibbing about being an exclusive nurser. Now that I have read this thread, I see that there are several women who were in the same boat that I was- and some who got AF back even earlier!