Usually as soon as there is a separation from the child is when the NCP should start to pay support. Of course the custodial parent has an obligation to go to court to collect it.

If the father has custody and is the custodial parent they wouldn't be paying at all.

The father of a child is obligated legally to start paying as soon as there is a court order for them to pay.

Legally, when a court orders it. Ethically when you separate.