Even when the baby can roll over on their own it's still best to at least place the baby on their back until 1 year of age.
But if the baby does roll over on their own then you don't have to move them back to their back and they can safely sleep in any position they roll over too.
Babies can usually roll themselves over by age 6 months but some may be able to roll over on their own by 4 months of age.
By then the baby can roll themselves back over but if you're worried still you can place them back on their backs but it's not required.
Just avoid putting any soft toys or blankets into the crib with the baby until they're at least 1 year of age.