Periods can start again at any time. They often start 5 - 6 weeks after you swap to bottle-feed or when you combine bottle-feeding with breastfeeding.
If you continue breastfeeding though, your period might not re-start until you stop altogether because breastfeeding is known to interrupt the ovulation process.
It’s hard to predict when your periods will start again as it takes a while for your hormones to settle down. It might return after a month of giving birth or not for another year. Both are normal.
If you're worried, speak to your doctor and also be aware that even if you're not menstruating there's still a chance you might get pregnant if you're not using contraception.