Onset of ovulation after giving birth?

"6 weeks - 6 months": Varies greatly and can range from few weeks postpartum to several months or even a year later. Don't rely on lactational amenorrhea for contraception!

When its ready: The body clock suppresses egg release during pregnancy through a complex interplay triggered by the hormones of pregnancy. Breast feeding can replicate some of these messages for some people but not all. Whenever the body decides it is ready, it resets the egg release pattern & it starts. It can/does happen before menses returns. Plan accordingly.