Pumping is an option for mothers who can't breastfeed or have to return to work.
If you cannot breastfeed your baby immediately after birth for any reason, it is important to start pumping within a few hours after birth. This will let your body know that it has to start producing milk. Pumping every two hours tells your body that it should produce milk every two hours (similar to a newborn's feeding schedule).
It is more important to pump frequently than to pump longer at each session. 10-15 min per session is usually enough though it might take much longer for some moms.
There is a lot of published literature on the guidelines for storing mother's milk. At room temperature, milk can be stored for 6 hours, in a fridge for 3 days, in a freezer with a separate door for 3-4 months, and in a deep freezer, for a year. Spoilt milk will smell bad. Feeding the baby spoilt milk might result in an upset stomach, and cause other complications. Play safe.
Make sure that you store pumped milk in the proper containers (clear plastic, glass) as that can affect the quality of milk and the preservation of the ingredients in it. For long-term storage, date and store in freezer bags specially designed for storing mother's milk.
Personally, I think it is better to give the baby the freshest milk possible.
Freshly expressed milk has live cells that protect the milk from getting spoilt and living cells that provide immune support. These cells drop in count once the milk is refrigerated/frozen. Formula on the other hand, doesn't contain the live cells and hence will spoil faster. Refrigerate any prepared formula and use soon, and discard any that is leftover after feeding the baby.
