It all depends on how it is stored.
Although eaux-de-vie can be kept in oak casks for several decades, once the cognac has been bottled, a few precautions are necessary to ensure it keeps well. Ageing in oak casks is an essential, compulsory step in making cognac. The flavours and amber colour of the eaux-de-vie go on maturing in our dark, peaceful warehouses for anything up to 70 years.
Once it has been bottled, cognac stops maturing. Unlike wine, which can go on improving, cognac ceases to age when in bottle. Thus, a bottle of cognac can be stored for a very long time provided it has not been opened and the cork remains intact.
Storing the bottles upright in a dry, dark place will maintain the quality of both cork and cognac. Once the bottle has been opened, try to keep the contents away from air, as this could cause the cognac to deteriorate.
Transferring the cognac to a smaller container can help avoid this problem of oxidation. Following these tips will enable you to continue enjoying your cognac for several more months.
Please do not share with anyone under 18. Drink responsibly.