See, here's why I almost never vote in these polls--the answer isn't there.
kb, we've been discussing these very issues elsewhere, and the answer is: Sometimes, maybe, depending. The worst thing that can happen after opening, of course, is evaporation of alcohol and perhaps other volatiles. This can happen pretty fast if you leave the bottle standing around with no stopper in it. In normal usage, we have all agreed that six months to a year is pretty safe, and sometimes, under optimal conditions, a bottle will be fine for several years. The lower the level in the bottle, the faster the deterioration. However long you've had it, when it gets much below half full, resist the temptation to try to make it last and drink it up in reasonably quick order.
I've noticed that some whiskies don't taste too good immediately upon opening, and improve shortly thereafter.