As soon as possible. Tummy time can be started as soon as you come home with your infant. As long as your infant is awake and being supervised, you can place him on his tummy to help build upper body strength, prevent flattening of the head and increase awareness.
After stump. Of the umbilical cord falls off is a good starting time, but in this first weeks of life head control is not good and you'll need to assist and only for a very short time. Take it slowly, and make it fun. They may get frustrated initially, but they soon enjoy it as they learn to crawl.
Now. From within the first week of life, junior ought to spend some waking time on his or her tummy.