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?

WHO recommends exclusive breastfeeding (i.e. no other fluids or solids) for six months and then continued breastfeeding combined with solid foods for 12-24 months or as long as mother and baby desire. Personally i breastfeed my baby until she is 15 months and she is on formula milk ever since then. But of course it is a personal choice. There is no hard and fast rules as to when you should stop breastfeeding.