Just like wine, sake has a very short life span once the bottle has been opened. In fact, like wine, sake should be consumed soon after opening to ensure that delicate fragrances and flavors remain intact. Although this varies from sake to sake, in most cases the more delicate and refined the flavor and fragrance of a sake, the sooner it goes downhill.
Of course, it will not spoil in such a way as to make you sick, nor will it turn to vinegar or become downright unpalatable. After a few days, however, sake in an opened bottle will lose its finely wrought edges and graceful curves. More solidly built sake such as junmai-shu, which has a bolstering acidity, may last a couple of weeks. But as a general rule, if a bottle cannot be drunk at one sitting, try to finish it off within a few days.
Naturally there will be occasions when this is simply not feasible. But fear not: There are a number of ways to use sake that has lost its zing.