Breastmilk, never frozen, can last up to five days in the fridge before use. Once used, it needs to all be used within a day or so. If it was frozen then thawed it should be used within 24-48 hours, generally speaking. Frozen then thawed breastmilk should never be left unrefrigerated, chilled milk is good for a couple of hours, and fresh breastmilk is good for up to eight unrefrigerated hours. As far as I know, formula is best used right away, same as breastmilk, but it should say on the can whether it can be refrigerated and for how long.
at 1:31 PM on Apr. 23, 2010