Glass of whiskey and a decanter on a dark surface
Here’s How Long Your Whiskey Will Last In A Decanter
The primary factor affecting how long whiskey lasts is how much it interacts with air. Whiskey in a decanter kickstarts the oxidation process, making it last around three months.
As you pour the whiskey into the empty decanter, it will swirl around, creating a lot of play between it and the air. This is what essentially kickstarts the oxidation process.
If you’re careful about how you fill the decanter, don't open it up very often, and leave it nearly full, the whiskey can stay good much longer, but this isn't an ideal method.
If you're planning on keeping your whiskey on the shelf for a long time, it's best to leave it in the bottle. Decanters are better suited for something that you drink regularly.