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.
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
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.
Ideally, babies should be sustained solely on breastmilk for the first 6 months and solids can be introduced without cutting down on milk feeds up till 1 year old. After 1 year of age, breastmilk still continues to provide essential nutrients and antibodies which will strengthen your child's immunity. Some moms switch to cow's or goat's milk instead of resorting to formula milk.