The Daily Portal Z site has an article which addresses the question of how long you can leave an open bottle of sake before you have to drink it up.
Writer Yoshinari Baba admits that although he struggles to drink a litre of water, he has no such problems with the same quantity of sake and so rarely faces the problem of what to do with an unfinished bottle.
However, if you work somewhere that sells sake it's a question you're often asked, as most people find it hard to get through a 720 ml 4-gō (4合) bottle on their own (never mind a 1.8 l isshōbin, 一升瓶). And especially tough on those who don't drink daily...
Do you have to drink a bottle of sake all in one go? Will it go off and become undrinkable after a few days? Baba says "relax". There's no problem in finishing off the bottle in your own time. Although there are a few caveats which he deals with later.
He points out that sake has no best before date or use-by date. The only date on the bottle is the year and maybe month of production, usually given as the Brewing Year (BY) that runs from July to the following June, following the brewing cycle.
As long as the storage conditions are good, for example in a fridge, Baba says there's no problem drinking sake even after several years. But there's no guarantee it will taste the same - the flavour will change almost daily after the bottle is opened. So his final answer is seal the bottle, keep in the fridge and finish within a week, but in theory you could drink it even after that.
At his own shop, Baba tastes each sake and places it on the SSI's two-axis diagram. (Image from the SSI International site.)
Taking a bottle of junmai muroka nama genshu Sanransei from the Mifuku brewery in Saga Prefecture, tasting it on opening he places it just to the bottom right of centre, a little on the full-flavoured side, a little on the less fragrant side.
Classing it as a rough sake with restrained flavour and aroma, and a fusion of sweetness, umami, acidity and scent... well, you can't express all of that on a two-axis graph, but the point he chooses to mark gives an indication of what you'll get.
After opening, in Baba's experience, most sake move left and down on these axes. The aroma settles, and the taste becomes lighter. Sake changes daily, so in the shop he makes sure to taste them every day to make sure they're still in good condition, noting this is part of the job for a kikisake or sommelier. The rate of change differs for each sake.
Of the sake he sells, the ones that change fastest will be noticeably different in just three days - most take a week for the flavour to change significantly. He sometimes removes a sake from sale and takes it home if it's still around after that time. (Although the bottles rarely last that long.)
He's also keen to point out that these changes can be positive - an overpowering or sharp sake can benefit from being opened for a few days and mellowing. Sake is alive, and if you examine it carefully, you can enjoy watching it evolve.
Finally, his tips for storage: keep in the fridge whenever possible. (And keep nama in the fridge even when not opened.) Some types of sake taste better when stored at room temperature, but when in doubt keep it in the fridge to be safe. There's a very small chance of microorganisms growing on the mouth of the bottle, which can make it taste extremely acidic. So keep the bottle as clean as possible. And keep sake away from light, which will cause the flavour to deteriorate to the point that it becomes undrinkable. [Very tempted to experiment with this, although not keen to sacrifice any sake for it!]