The timing of pumping while breastfeeding depends on the purpose of the pumping. For example, early in the breast feeding experience, some women need to pump before nursing the baby to help pull the nipple to a more accessible position or to help reduce some of the engorgement so the nipple is more prominent. Once breastfeeding is established, some women pump for comfort and others pump to store milk for future feedings. If the purpose of pumping is merely for comfort in moms with full breasts and sleeping babies, pumping or hand expressing just enough milk to make mom comfortable is best. It is important to remember that breast milk supply is related to demand. The more stimulus and emptying occurs, the more milk is made.
The most common long term reason for pumping is to build up a supply of frozen breast milk to use when mothers need to be away from their infants. In this case, it is best to initially start pumping after the baby has finished nursing. This will prevent limiting the supply available to your baby for the feeding and will give extra "demand" or stimulus for more production. Also, if your baby has a long sleep stretch, pumping about halfway between feedings will help build supply without taking away from your baby's current needs. Mother's that go back to work and want to continue nursing should pump as near to their baby's feeding schedule as work permits.
Remember, milk should be stored in the refrigerator for only 24 hours. Frozen milk can be used for weeks to months depending on the temperature of your freezer. Date all frozen breast milk and use the older milk first so that your milk does not go to waste. It is best to store the milk in smaller quantities and thaw out two smaller portions than it is to store the milk in larger quantities and have to throw some out. To maintain the immune properties of breastmilk, it should not be thawed or heated in a microwave. Thaw milk at room temperature between feedings or in warm water. Any milk that has been thawed but not put in a bottle that the baby is feeding from can be saved in the refrigerator till the next feeding.