When should I start my period after breastfeeding???

So my question is after you stopped or slowed down on breastfeeding when did you start your period again? My son is 9 months old I breastfeed and have yet to get a period. I've always just blamed it on breastfeeding, however when he turned 4 months I started him on baby food. Then when he turned 7 months i switched him to formula due to not being able to keep up with him. I still feed him at night but that's it. I don't pump at all anymore either. I didn't know if anyone had an experience like this and how long it took it to come back. Did anyone have to go to the doctor? I just took a pregnancy test just to make sure and it was negative.