Sake is an alcoholic beverage which itself has a bactericidal effect and can be stored for a long period of time. Though it seldom goes bad, it deteriorates in quality. It could be deteriorated in a short period of time depending on where it is stored.
One of the big factors is light. Not just sunlight but also light like fluorescent light could change the color and generate light-struck smell. Another factor is temperature. The higher storage temperature could cause odor, turbidity and zatsumi (bitterness or unpleasant taste) and the speed of this change is accelerated as the temperature gets higher. Also, severe temperature changes should be avoided. Sake is recommended to be stored in a fancy box or wrapped in newspaper to be stored in a cool dark place.
Generally, sake is pasteurized twice during a process from the filtration up to the shipment. It can be stored at room temperature, but it is best stored in a cool place. However, some types of sake are only pasteurized once, or not at all and these types such as namazake (unpasteurized sake) and namachozoshu (live storage sake) should always be put in a refrigerator, and the leftover, too.
Though maturing sake intentionally is one of our favorite things, if you are to savor the real taste, you should drink it off as soon as possible.