How long after you start getting stretchy CM should you start OPKing?

Gi

I am CD9. I just had my 2nd postpartum period. That cycle was 52 days. I don't know if postpartum hormones are still jacking up my cycle or not. (I've had long cycles my whole life though

. I thought it may go back to normal after having a baby.) I don't know when I should be starting the OPKs.