A. Well, it all comes down to protein and storage. When stored tightly and protected from light, medium all-purpose flour can stay in your pantry for about eight months after opening with no difference in flavor or quality. This is because the date printed on it is usually "best before" and not "use by", which automatically gives you some leeway. Low-protein flours like cakes or pastries last longer; High-protein foods like whole grains can become suffocating after about six months. The exception to the rule are self-rising varieties - the baking powder they contain loses potency at about the same rate as durum wheat. But overall, if you keep it in the fridge, you can buy yourself up to two whole years.