Different people have different sensitivities and where one might notice a deterioration, another might not.
The main enemy of whiskey is oxidation. About the only time this affects whiskey in a sealed bottle is when you leave a small amount (less than 25% in the bottom of a bottle. Even then, from my experience, it takes years to turn. I try to avoid having such a bottle sit around too long and have never, in my experience, had one turn on me. I have tasted bottles that have turned in other circumstances, but I have never had one in my control turn.
The point is, acute sensitivity aside, a well-sealed but only partially full bottle of American straight whiskey will be "good" for at least a year.