Recommendations on breastfeeding from some of the leading organisations:

The American Academy of Pediatrics (AAP)
“The AAP recommends that babies be exclusively breastfed for about the first 6 months of life. This means your baby needs no additional foods (except Vitamin D) or fluids unless medically indicated. Babies should continue to breastfeed for a year and for as long as is mutually desired by the mother and baby.”

Source: http://www.aap.org/breastfeeding/faqsBreastfeeding.html

The Australian Breastfeeding Association (ABA)

“When to wean – This is a very personal decision, which must take into consideration the needs of baby and mother, as well as the home and family situation. You may want to, or you may have to, wean early. Try to make it gradual if this is the case. On the other hand, you may want to wean later – into your baby’s second year, or later. Ideally it is best not to set a time limit for weaning unless it is absolutely necessary.”

Source: http://www.breastfeeding.asn.au/bfinfo/weaning.html

World Health Organisation (WHO)

“WHO recommends – WHO strongly recommends exclusive breastfeeding for the first six months of life. At six months, other foods should complement breastfeeding for up to two years or more.”

Source: http://www.who.int/features/factfiles/breastfeeding/facts/en/index.html