I understand IPAs are less amenable to aging than, say, a barely wine. But how long, specifically, can you age an IPA - days, months, a year or two?
DIPAs generally have a high enough ABV (7%+) to age for a few months...but you probably don't want to.
Most contemporary IPAs and DIPAs are best drank within 3 weeks from the date of bottling. Stone's "Enjoy By" Series gives you 5 weeks to drink the IPA if properly refrigerated. Super hop-bursted IPAs with a ton of aroma like Heady Topper recommend to drink them within 2 days, claiming the flavor and aroma begins to degrade after only a few hours if unrefrigerated.
One notable exception is Dogfish Head's 120 Minute IPA which is super high ABV and can probably be considered a barleywine for the purpose of aging.
Less intense IPAs like Long Trail's Vermont IPA could probably keep up to 2 months, maybe 3 with really good refrigeration but not much beyond that without starting to taste real strange and losing a lot of their floral and herbal notes.
Sorry for the late response, but as I look on the internet for discussions concerning this topic I ran across this question.
In the mid 1800's in England, IPA's were created and produced for multiple reasons. Long story short, as Europe was going through a technology renaissance period, ingredients used for brewing changed and grains that were converted into malt became lighter. Therefore, dramatically changing the "beer", that was much darker at that time, now into and ale. As this changed the palettes of many at that time they also introduced a great amount of fresh hops to these new ales to make a strongly hopped ale. The term IPA was introduced as a well known brewer(s) started shipping this new type of beer to India, giving it the distinct name IPA-India Pale Ale.
I am providing this back ground to make a point. As this product was fermented, it was very typical to age and IPA Beer for a minimum of 1 year. This was done to assure that the beer was completely fermented and safe to transport on ships to India. This trip would take even more time and put the produced under various environmental stresses. This beer or "ale", was hopped very strongly and even topping the very hoppiest beer on the market in the U.S. today. Using the freshest malt as well as the freshest hops.
My final point or argument is, that as I recreate the IPA and use fresh ingredients typical to a real IPA, I find that my IPA's last over a year in the bottle. As I drink them 3 to 6 and even 9 and 12 months the hoppy flavors still remain and even develop.
There are many variables that allow me to get to this end result. This including yeast and malt selection, and also multiple methods that help produce beneficial fermentation, allowing the taste of the final product to be clear of flaws that would be created by the yeast during primary as well as the bottling (carbonation) period.
After 30 years of beer making many of these truths become evident as I continually taste the final product, take very good notes and adjust my recipe.
Finally, I can say that with the correct procedure of beer making, an IPA can be enjoyed at various stages from 1 week and even up to one year and can still be enjoyed for its distinct hop flavor and aroma.
I agree with the other answers: IPA's are not really intended to be aged like a barleywine. However, the style was historically brewed to survive the long journey by ship from England to India. So while they might lose some the freshness and hop volatiles over time, you can certainly keep them for many months and they will still be very drinkable. (Assuming, of course, that the brewery handled the beer properly prior to bottling).
So, don't be in a rush to finish them. And never, ever throw beer out that you think may be too old without tasting it first. Beer does not spoil in the sense that it becomes harmful or dangerous.