You can also put marbles into them to restore the volume, but I don't bother. I've only once had a bottle that went far off (meaning too much of the alcohol evaporated out), and I'd kept it with only 2-ish shots in it for probably a year and a half. So normally after I get down to about the last quarter bottle, I try to finish them within a couple months, and that's done me well.
For me, Laphroaig Quarter Cask was also the first Whisky I bought. I kept it around for a long time (maybe 2 years) because I did not want to "waste" it too quickly.
The last few glasses I drank were really disappointing compared to the initial fanstastic glasses. I can't describe the tastes, but it was simply not good anymore. When too much air gets into the bottle, the Whisky can oxidize, changing its taste (usually for the worse).
From what I've seen, most people using preservation techniques aren't worried so much about alcohol loss as various other molecules getting oxidized, changing the flavor profiles they spent a lot of money for. (I don't have enough experience to say how valid that is, but it's certainly a problem for wine.)
