Should I worry about evaporation if I have bottles open (obviously with the cap/cork in) for extended periods of time? Say 1yr or more?
ooh ho hoo, can of worms! One could write an entire book on the subject... and I think there's a thread dedicated to this buried somewhere deep in the unsearchable forum. The bottom line, though, is "dunno".
Some bottles last for years after uncorking (and can even improve over time). Others are a hot mess in just a few months.
However, evaporation isn't the problem, oxidization is. Your whisky might not disappear, but it might go bad... well, not bad, per se, but neutered certainly.
In general, I think most bottles would be fine after a year, but it depends on the circumstances. (I once accidentally left a bottle in a hot car for an afternoon and it destroyed the whisky completely.) Independent bottles, cask strength, single cask stuff should last a good long while, but others like Diageo-based standard product lines maybe not so much (but only because they often don't treat their whiskies with enough respect).
- proper storage (this goes for the unopened as well). cool, dark place, bottle standing upright.
- the more O₂ the faster it deteriorates, so the higher the contents in the bottle, the longer it will last.
- there is some debate about whether colourant and chill-filtration are detrimental to longevity. I'd err on the side of caution and assume they do.
- I think higher strength might also add to longevity.