How long can you keep whiskey in the freezer?
You have no problem at all. 


Whisky doesn't spoil, it doesn't need to be kept in the refrigerator or freezer to preserve it, the same is true for many other spirits like gin, vodka, tequila, etc. Some like to keep spirits in the freezer because they like to drink them cold, it doesn't make them last longer
Storing spirits in the freezer won't damage them because alcohol freezes at a much lower temperature than water. 80 proof spirits will freeze at about -16F (-26C), freezer temperature is usually about 0F (-18C), so unless your freezer is extremely cold your booze won't freeze. If your whisky did freeze it could ruin it, but it's very unlikely