As simple as breastfeeding and pumping is in theory (boob + pump or baby = milk) it's often a bit more complicated than that. Especially if you don't plan to use the milk immediately after you pump it. You are then met with questions like "how long does frozen breast milk really last?" and "how can I make sure it doesn't go bad?" Both valid and necessary questions to make pumping and storing your breast milk as simple and hassle free as possible.
Like most foods, breast milk can't stay frozen forever. According to La Leche League International (LLLI), it's best to use your frozen milk within three to six months of freezing it if you're storing it in the chest section of your freezer. However, if for convenience's sake, you choose to store your breast milk in the freezer door, LLLI noted that it's best to use it within two weeks due to the changes in temperature from opening and closing the freezer door.
To give your breast milk the longest "shelf life" possible, Mayo Clinic suggested you store it in a deep freezer, which is known to have colder temperatures and can prolong the milk's life for up to a year after freezing it. It's important to note that once you've set your milk out to thaw, it can't be refrozen, according to Baby Center. Doing so breaks down important nutrients in the milk that your baby needs.
It may be most convenient to have a few bags of milk in your main freezer, ready to go as soon as you need them. Then the newer, more recently pumped milk can be stored in a deep freezer. As you need more, simply move it from the deep freeze to the regular freezer to free up room, and ensure you're using the older milk first before it hits it's "expiration date."