You can actually start expressing as soon as your baby is born. This is usually recommended if the baby is born pre term and may be in a special care unit separated from the mother. Studies have shown that expressing on the first day, within the first 6 hours or as soon as mothers are able to will greatly increase long term milk production.
For mothers who usually breastfeed, and are returning to work, expressing should start at least a week before. This is so that the baby can get use to receiving the milk in a different way. The baby may not accept the milk initially as it is not being directly breast fed. However, most babies do adjust and continue to feed well.
If possible, you should express around every 3 hours, including once in the night when prolactin levels are highest, aiming to express 6-8 times in a 24 hour period. Remember that breast milk is made on a supply and demand basis, and the better your baby breastfeeds, or the more often you express, the more milk you will make for your baby.
An Australian study found that women who express milk are more likely to continue breastfeeding up to 6 months.
You can hand express, use a hand pump or electric pump. Speak to your midwife or health visitor about the methods and techniques for each before you decide which to choose.
The expressed milk must be stored in sterile bottles (or sterile containers with a lid). Remember to label each bottle or container with your name (if your baby will be with others at a nursery), the date and the time you expressed it. You can keep expressed milk in the fridge for 48 hours. But not in the door of the fridge, as the temperature fluctuates too much. It can also be stored in a freezer for 3 months.
Ref: Breastfeeding duration in mothers who express breast milk: a cohort study. Nwet N Win School of Public Health, Curtin University, Perth, Australia