From birth to six months
You should try to exclusively breastfeed your baby for the first six months (26 weeks) of its life, as breastmilk has all the nutrients he/she will need during this time

At six months
From about six months, your baby is able to eat solid food. At this point, he will need more than breastmilk alone can provide. You will need to wean your baby around this time – introducing a range of 'non-milk' foods gradually, until your baby is eating the same foods as the rest of the family.

From six months to one year
Babies need breastmilk alongside solid food from six months until they are at least one year old. If you are not breastfeeding, you should continue to give your baby infant formula milk.  Cows' milk, as a drink, is not suitable for babies under one year, although it can be used in cooking. As your baby eats more solid foods, the amount of breastmilk he wants will decrease. Once he is eating solid food three times a day, you may find he wants to breastfeed less often.

One year plus
After one year, babies can continue to breastfeed, or change to drinking whole cows' milk. You can continue to breastfeed for as long as it suits you and your baby, and it will continue to have benefits for both of you.

(Source: NHS.gov.uk)