Ideally, I should breastfeed my baby until when? And milk will be the main nourishment until when? is it necessary to offer them formula milk after certain period of time?

I think the answers above sum it up well... Formula milk is only necessary if they can't get enough breastmilk. It's not the end of the world to use formula if you need to but many prefer to go with breast milk exclusively and then have cow's milk starting at 1 y.o. Having said that sometimes some moms get sick and need to use formula milk to supplement and that's perfectly okay. Talk to your doctor if you need to do that to be sure. If you're not sick usually the mom's body automatically produces enough milk