Unopened Bottles - Sealed and stored away from bright sunlight, in a cool place, you should be able to keep a single malt indefinitely. The corks used in today’s bottling are exceptionally good and there should be no deterioration. However, in much older bottlings, before corks were made to current high standards, the corks have been known to deteriorate and spoil the contents.

Opened Bottles - The answer really depends on how much air is in the bottle. As you take each dram, more air is then able to interact with the spirit. The more air there is, the greater the chance that it will start to alter the character of the remainder of the spirit. A little left for too long will result in faster deterioration.

I've decided to buy glass marbles, run then through the dishwasher and carefully add them to the bottles as I drink them. I'm hoping that slows down the deterioration.