According to BreastNotes.com, a woman continues to lactate until she stops expressing the milk from her breast. A woman is able to produce milk for nearly 20 to 30 years if the milk is a constant need.
BreastNotes.com also points out that women don't have milk in their breasts until they go through pregnancy. Once the milk is no longer being expressed from the breasts, women stop lactating; however, women start lactating again if another pregnancy occurs.
Expressing the milk from the breasts happens as often as the newborn infant needs to feed, according to BreastNotes.com. Most women continue to produce milk as long as expressing happens every few weeks.