Ideally, an open whiskey bottle should be consumed within three to six months of opening, but there are other factors to consider that may influence your particular bottle.
Whiskey bottles are tightly sealed to avoid tampering and leaking and protect the bottles during transportation. However, the primary purpose of the seal is to prevent oxidization.
Once a bottle is opened, the whiskey within meets air and thus oxygen. With prolonged oxygen exposure, the whiskey takes on unpleasant flavors described as earthy or graphite-like.
To prevent oxidization once you open a bottle, decant the whiskey into a smaller bottle with an airtight cap, decreasing the amount of air it's exposed to inside the container.
Snap-top bottles are also a good option for preserving whiskey. Whichever container you choose, keep it in a cool, dry place away from heat and sunlight, like in a kitchen cabinet.
With all that said, if you really want to get the most out of your whiskey, be sure to consume the entire bottle within two months of opening — no matter how you’ve stored it.