You can start the baby out on tummy time when the baby is 3 months old however some babies may not be ready for tummy time until 4 months of age.
But you can try tummy time around 3 months of age and let the baby do tummy time for around 30 minutes or longer and just keep an eye on them at all times which you should always do.
You can keep doing the tummy time until the baby learns to roll over on his/her own which is around 6 to 7 months of age.
My son started tummy time at age 3 months and within 6 months of age he was able to roll over on his own and he enjoyed tummy time.
I did tummy time with my son when he was a baby 2 to 3 times per day when I could be available to watch him.
He's now 8 years old which I can hardly believe as it seems like only maybe 1 year since I had him but wow it's been 8 years.