My experience is that when you store your whisky in a cool (NOT cold) and dark place AND you use a good cork you will be able to store the whisky about 6 months before changes in taste will occur.
It also depends on the ratio whisky:air in the bottle, the emptier the bottle, the faster changes will occur.
Sometimes when it's been a while since you last had a drink from an already opened bottle it seems that the taste has changed. But if you add just a little bit of water (a drop or 2 will do fine) and let the whisky 'breathe' for a minute, it will usually recover from the long and quiet period in de cold and dark place it has been.
[This message has been edited by MaltMan (edited 04 July 2002).]