When to throw your dog food away is an interesting article which appeared in Dogs Naturally magazine. It is about the deterioration of fats in dog food once the bag is opened. It is a bit of a dilemma with small dogs because big bags are cheaper (sometimes markedly) but they can last some time - too long perhaps.
NB For those who use cold pressed dry dog food, the storage rules highlighted in the links do not apply. Cold pressed food should not be stored in airtight conditions. It should be left in the bag in which is provided by the manufacturer.