Keeping opened bottles over time
A whisk(e)y or other liquor can last a long time under the proper conditions, with the key factors being, as Jojo said, keeping it out of direct sunlight, at a cool, constant "room" temperature in the mid- to high-60's, and is kept from contact with air (oxygen). In my opinion, the hardest of these conditions to control is sealing adequately from air contact (oxidation). Even sealed bottles can eventually leak and evaporate because old seals (especially cork, but more modern ones as well) fail.
That said, there are some newer and relatively simple and inexpensive ways to keep air from getting to the liquor. These involve either vacuum storage or use of inert gases, usually argon, to keep air from reaching the liquor, and both are commonly used for wine.
The vacuum method simply uses a tool commonly available at quality wine shops to create a vacuum that keeps air out. This obviously implies a vacuum seal, so if the seal fails, so goes that idea.
The idea behind argon or other inert gas is that (a) the gas is heavier than air and will displace the air inside the bottle and keep it out; and (b) being inert, by definition it won't interact chemically with the liquor, rather floating over the surface, with surface tension of the liquid keeping the gas away from the liquor.
To keep any taste from interfering with the liquor when you open the bottle for a nip, I would wait 15-30 minutes after opening the bottle before pouring a dram. Since the gas is heavier than air, you might even want to tilt the bottle a bit to allow it to escape. Reason for doing this is that I've heard some people drinking directly after opening complain that the gas intereferes with taste -- but that's because they didn't follow these instructions.