Formal tummy time can start at about two or three months—or as soon as baby can lift her head. When you do start, baby should be awake and supervised, and you should keep it to 30 minutes a day, max. The truth is, it’ll be a miracle if you can get baby to stay tummy down for 30 minutes, so don’t get discouraged. Try it off and on throughout the day to avoid stressing her (and you) out. Tummy time is so important because it strengthens the neck and helps develop baby's motor skills; plus, it wards off a condition called positional plagiocephaly (or flat head), which some babies can develop from long hours spent on their backs.