How long will Sake last in the fridge?
Unless contaminated by, say, drinking straight out of the bottle, it will likely be safe to consume, but it will probably have deteriorated in quality.

Sake which is no longer appealing to drink may be suitable for use in cooking; if you're not accustomed to cooking with sake, consider something simple, like potatoes simmered with a modest amount of water, sake, salt &amp; pepper, soy sauce and butter.

Low-alcohol beverages, whether fermented or slightly fortified, tend to deteriorate in quality over time, although they will become stale less slowly if their containers are sealed airtight and chilled. Port, marsala, sherry, vermouth, wine and sake all typically best consumed within a short time after opening, although the specific chemistry may vary.

Some folks consider a sake that's been open, unrefrigerated, for more than a few weeks undrinkable. I don't know if I'm that rigid, but it's a good benchmark. I've seen certain sakes do ok for a few months in the refrigerator.