How Long Will an Opened Bottle of Whiskey Stay Good?

I'm fairly new to the scotch world, but have still managed to acquire and taste a few good ones. The problem is when i get a new bottle i can't help but open and try it. I now have about a dozen open bottles of whiskey. Recently though it occured to me that the spirits may not "keep" indefinately.

How long will an opened bottle of whiskey stay good? My favorite scotch is the Macallan 12yr and i drink it on a regular basis to wind down after work. However i also have a bottle of Highland Park 25yr that i don't necessarily want to drink on a regular basis, but it would be nice to have for special occasions-- such as maybe an annual new yrs toast. Does whikey go bad??

Thanks guys...

Seth