You can start tummy time from day 1, and you should do at least five minutes a day. It doesn't have to be in one big chunk, though, it can be a minute here and a minute there (but more is good, too). And things like having them lay on their tummy while they're on your belly count as tummy time. When they're littler, this is a good way to get started, as babies like looking at faces and then they aren't ending up with their face just smashed into the floor, and it is just a more pleasant experience for everyone involved.