I have read from many sources that 12-18 months is optimum. At least 6 weeks for colostrum. I nursed my children from 10 months upward. Our pediatrician is amazed at how healthy they are. My four year old never needed an antibiotic until this week -- which was the first prescription any my children have had in over 4 years, except for a stool softner about 3.5 years ago for my second oldest.
We are expecting our sixth child in the spring, and I plan to nurse him or her at least one year. I do not soley nurse that long, adding rice cereal and other foods as the baby grows and requires more calories and solid foods.
According to American Pediatrics, babies benefit greatly and they recommend exclusive breastfeeding for the first six months of life. Furthermore, they encourage, "Breastfeeding should be supported by your physician for as long as it is the right choice for you and your baby."
For more information from American Pediatrics, please visit the Source: http://www2.aap.org/breastfeeding/faqsbreastfeedin...