I've copied this from post (with slight augmentation here) I responded to earlier today:
Very few in the industry know enough to submit to the fact that spirits do go bad once opened. However, the do go bad or can have negative consequence over time!
Depending on filtration, cask marriage, storage conditions (and this means from bottling, transport to market, time in distribution warehouses and retail shelves to the drinker's bar/home environment, hot/cold/light exposure) spirits can change negatively in bottle. Given a good bottle at opening, once a bottle is opened and has air contact, unless it's being preserved in a no-oxygen environment like argon, you have 3-6 months to drink it at its best. Sometimes just 2-6 weeks for more unique single cask-type bottlings. There are certain aldehydes that develop with air exposure that can, and do, negatively affect the spirit in time.
Most vodkas hold up longer because they are quite neutral and have had most flavor congeners removed in the multiple distillation and filtration processes compared to most whiskies (especially single malt whisky). So understand, malt whiskies with hundreds of congeners can negatively evolve with air contact much faster.
The problem is most people in the industry don't know this, and can't recognize it if they do! In fact, after 12 years in the industry and 30 years studying products, I've only met a handful of people who know this and purport it in their business dealings.
Phillip Hills' book Appreciating Whisky goes into this in some detail. He is the only one I've read who's drilled this point heavily!
The fact is, big peated malts like Lagavulin have so much going on in them that there is a lot that can go wrong, but people probably notice any issues in them less because of all of the things going on compared to say a bottle of Dalwhinnie or Glenkinchie that's been sitting open on a bar's shelf for three years.
My point being, I think you had a big lesson that your malt went south in 3-4 months!
My advice, only keep open what you can drink in 2-3 months (at least you only lost 1/4 of a bottle). Being that you had your whiskey open about 3-4 months, your experience is one that proves my point. And where I've had bottles open longer without incident, why risk it?
I get through 6-10 bottles of whisky a month so I routinely will have 8-12 bottles open at home. I've toyed with buying a preservation system but now simply mark the date I open a whisky on the bottle's label and drink up in a timely manner. I call it self-preservation.
Living near New York, I have frequented most of bars with better whisky selections. None of them have anything in place for product preservation. I've subsequently had terribly drams poured me at Brandy Library, Keens Steakhouse, St. Andrews, Caledonia to name a few. I've also had the same issue at whisky bars and pubs in Scotland. The only two establishments that have ever done anything when pointing out a bad dram is the Highlander Inn in Craigellachie and the Mashtun in Aberlour, they removed the bad bottle from the shelf, didn't charge me for the bad dram and got me another whisky. So rather than try a rare whisky that's been open for a year or more in a pub, I tend to drink what's been opened most recently when I'm out.