Tummy time

Do it from from birth.

Do it for as long as lo is content so not too long at first.

Dylan hated it at first but got used to it. It is really important for development and it will help later on with crawling.

When they get a little older put toys infront of them and they will enjoy it more.

As for how much - as long as they want (or can handle) 5 min is fine to begin with but don't force it if they don't want to, just turn them around after every nappy change for a couple of minutes

uhm oops I didn't know that I am supposed to do this and my son is almost 4 months old...

You can start from birth. Even laying on mommies chest when mommy is laying down is considered tummy time

At what age do you have to start with it and for how long per day?

Both my kids hated tummy time so I didn't force it.

Its very NB for neck and back muscle development, its when baby spends a certain amount of time each day laying on their tummy