Prolonged exposure to heat or direct light can lead to spoilage why it is best to keep sake refrigerated in a cool or dark room. In most cases you have to consume it within a few months after you bought it.

An opened bottle can be stored in the refrigerator for weeks but if you want to get the most out of the full flavor it should be drunk within 2-3 days. The reason is once premium sake is opened it begins to oxidize. Probably opened sake is drinkable much longer, it depends a lot on the quality of your purchase.