Once opened, sake oxidizes but fortunately more slowly than wine.
Drink sake within one week of opening but the most pleasurable state of the sake will be in the first 3 days.
Unopened, sake is best drunk within 12 months of the bottling date or 2 years if kept in cool storage/refrigerated. Of course this time is way shorter for Nama (unpasteurized sake) or if the storage area gets warm. The cooler temperatures will slow down maturation.