Tummy time, how much? Start early, build up.

“Play and interact with your baby while they are awake and on their tummy 2 to 3 times each day for a short time (3 to 5 minutes).
You can start doing tummy time the day your baby gets home from the hospital. ... Gradually increase the amount of tummy time as babies show they enjoy it. Work up to 15 to 30 minutes each day by 7 weeks.” American Academy of Pediatrics

“Most babies can start tummy time sessions a day or two after they are born. Babies benefit from having two or three short (3- to 5-minute) tummy time sessions each day. As your baby gets older, you can have longer, more regular sessions throughout the day. Pediatricians recommend that by about 2 months of age, babies be getting 15 to 30 minutes of total tummy time daily.” NIH Safe to Sleep

“Tummy time is important for your baby's development as it helps to build the muscles your baby needs for sitting and crawling. You can start doing tummy time from birth by lying your baby on your chest – but only do this when you're wide awake and unlikely to fall asleep.
Little and often is best to begin with. Gradually increase the amount of time you do this day by day.” NHS

“Tummy time builds head, neck and upper body strength. Your baby should do it often each day.
Start tummy time soon after birth. In the first few weeks, try tummy time for 1-2 minutes. Build up to 10-15 minutes several times a day.” Raising Children Network

“An awake adult should supervise awake tummy time. This helps with your baby's motor development and prevents flat head syndrome. Start with a short amount of tummy time soon after you get home from the hospital. Gradually increase the time until your baby is doing at least 15 to 30 minutes of tummy time every day by the time they're 7 weeks old.” American Academy of Pediatrics

“Try to include at least 30 minutes of tummy time spread throughout the day when they're awake.” NHS

“Babies should also have supervised “tummy time” when they are awake, for 10 to 15 minutes, and at least 3 times a day.” Canadian Paediatric Society

“Be physically active several times a day in a variety of ways, particularly through interactive floor-based play; more is better. For those not yet mobile, this includes at least 30 minutes in prone position (tummy time) spread throughout the day while awake.” World Health Organization

“Tummy time is for babies who are awake and being closely supervised. It helps them build strong muscles and skills they need to reach important developmental milestones.” American Academy of Pediatrics

“Supervised tummy time is important because it helps to:
Strengthen baby’s neck, shoulder, and arm muscles so the baby can start to sit up, crawl, and eventually walk on their own ... Prevent flat spots on the back of baby’s head” NIH Safe to Sleep

“Lay your baby on his tummy when he is awake and put toys at eye level in front of him. This will help him practice lifting his head up. Do not leave your baby alone.” CDC