Depends. This depends on how you define it. Babies can arch their head and flip over accidentally almost immediately after birth. You must be prepared and never leave them on an elevated table where they could flip off. Purposeful rolling requires they reach 3-4 months in developing the shoulder roll.
3-5 months. Most babies begin to roll over around 3 months and get quite good at this new skill by 5 months. Generally babies will first roll from their stomach to back and then back to stomach. Rolling from your back to tummy takes more neck and arm muscles so it comes later. And the next milestone to look forward too after mastering rolling over is sitting up!
About 4-6 months. Babies typically begin to roll over at about 4 months, however many babies will rollover by accident much earlier than this. Never leave leave your baby unattended your bed or changing table, even for a minute. Give your baby plenty of tummy time and you will see her get stronger every day. If she is not rolling over by 6 months, talk to your pediatrician about it.