The packages have expiration dates on the outside (the foil/plastic part). If you let it go too far past that date, I've discovered the milk starts to separate and can sometimes come out in clumps, or only half of the milk comes out. For the most part, they seem to last for at least several months - I wouldn't go past a year, though... and probably less. It just depends on the individual expiration dates. They're on the side of the outer packaging.
6 years ago