open about 5-6 years..
I have bottles of Rock Hill Farms, The Balvenie, Remy-Martin XO, and Courvoissier that have been nursed for years. The two cognacs were purchased in Martinique in 1990.
I rarely do this, either finishing them fairly quickly or blending them. However, I have two ryes I've kept about a year in a warm part of the room and both are affected IMO by oxidation, that tell-tale "earthy/dirty metallic note". They were half full, too. Recently, I tasted from a number of bottles in an office after work, a friend of mine, and had the ditto experience. I could tell the bottles had been opened and closed quite often and noticed that he didn't close them until the session was over, which presumably was the practice on his previous tastings. This has lead me to think that unless the bottles are kept on the cool side, well-sealed and not opened constantly, there is a real risk of oxidation. I think temperature may be the most important factor.
The simple reason this does not happen in cask in a hot warehouse is, IMO, the much higher alcohol content of the cask, and the wood environment, all that tannin and char.
I have a dozen or so bottles open right now, which is way too many (would prefer maybe 4-5). Of those, I plan on finishing 10 within the next year. The other 2 are definitely special occasions-only. They're both still quite fresh and have been opened less than 6 months, but I've done my best to make them last. Used Nitrogen spray and they're sitting down in my basement storage in complete darkness with a temp range of 50-70 F throughout the year.