The return of your period after giving birth can be highly unpredictable, especially if you're breastfeeding but even if you're not. I've read that if you are NOT breastfeeding, your period will probably resume between 4 to 10 weeks after giving birth.
The tricky thing is that you will ovulate BEFORE your first period, so there is no way of telling when you first become fertile again. To avoid any surprise back-to-back pregnancies, be sure to USE BIRTH CONTROL right away and consistently after your baby is born. Check with your doctor about what method of birth control is best for you, especially if you're breastfeeding.