This is a heated topic of debate among whiskey drinkers, actually. The heart of the answer lies somewhere between the sensitivity of the palate and the length of time that a whiskey remains open... You may hear that leaving the whiskey in an open bottle drastically changes the flavor, subtly changes the flavor, or does not at all change the flavor depending on who you talk to and where you look.
But, it's obvious that SOMETHING happens to whiskey after the bottle has been opened, right? It's the process of aeration - the same process over which so many wine enthusiasts debate. As air passes through, water and alcohol slowly evaporate from the whiskey... thereby leaving behind a stronger concentration of the other substances in the liquor. For very fine whisky, I have been instructed at distilleries to transfer to smaller (airtight) bottles if the opened whisky is to be stored for very long - in order to prevent a change in flavor or potential loss of aroma.
Personally, I can taste a difference in certain bourbons within a couple weeks of being opened (I only intentionally aerate when it's in the glass in front of me in order to get some of the extra aroma out of it.) But, I don't really taste a difference in scotch after the same period (of which I admittedly drink less... unless you're offering ;)
I'd say after a year, it would be difficult to argue that something about the flavor hasn't changed (but that may, again, vary by palate.) However, my whiskey never seems to last that long, be it bourbon, scotch, or rye haha