As the others have said it varies from woman to woman, personally i got AF back when Tarleha was 6 weeks old (i only had post-partum for 3 weeks). Personally i wouldn't want to wish them to arrive as i found them worse than before pregnancy.
It doesn't hurt though to get it checked out, i was the oppisite and went to make sure it was my period as i wasn't happy that it arrived so soon.