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?

Like what the rest have mentioned, it is recommended to breastfeed a baby for at least six months to help provide the required nutrients and build up his/her immune system. Solids are usually introduced when the baby is around 6 months old (or even 4 months if the baby appears ready). Still, it would be good to continue breastfeeding since the child may not be able to get the appropriate (and balanced) amount of nutrients when first starting on solids. Quite a number of my friends try to breastfeed for as long as they could. A few started introducing their babies to regular milk after their babies are one-year old. For those who had to stop breastfeeding before their babies are one, they supplement their babies' diets with formula.