How long should I condition an IPA?

I don't really condition dry-hopped beers like IPAs or German wheat beers. The hop aroma of an IPA or typical wheat beer aroma masks a bit the harsh off flavor of young beer. The dry-hop or wheat beer aroma does usually decline with time. Therefore, I recommend drinking them fresh.

For an IPA, I dry-hop for one week when the main fermentation is over (usually three days after pitching the yeast). After dry-hopping, the beer can be filled to kegs or bottles. With sugar, the carbonation takes around one week. From now on, the beer is drinkable. Usually, the aroma of an IPAs improves a bit over the first few weeks and declines then.

If you're kegging the beer, age it until it passes a diacetyl test. This might not actually mean waiting at all, but if it is chilled before that cleanup is done, you might have an unpleasant surprise later.

Bottle conditioning seems to be more forgiving since the second bit of fermentation helps things get cleaned up.

After that, its just matter of taste, as others have said.

I agree with the advice given in the other answer that your own judgement tells you when the beer is ready. However it's still possible to give an estimate: I'd say 5 weeks from brewday for low alcohol, under 4%, 7 weeks for medium - 4-6.5% For big beers, you'll need to see how the beer cleans up.

As others have said with an IPA, on the one hand you want to drink it as fresh as possible to preserve the hop flavors and aromas, but also want to give enough time for the hops to blend suitably.

While it's true that there are no hard and fast rules, you can follow the rule of thumb that you should age for 1 week per 10 gravity points. So for a 1.065 you'd go for 6 weeks. I've used this for quite a while now after it was suggested to me and I've always had good results that don't taste green.

It's worth taking into consideration what others have said about IPAs and drinking them young though. With all of my IPAs I taste them at 4 weeks, as I've found through previous brews that the hop character seems to be at its best then (in my opinion), so I'll often just start drinking it at 4 weeks regardless of the OG.

First of all, there is no rule about time for beers. The beer makes its own schedule. In terms of aging, there are no rules either. The beer is ready when it tastes ready to you. I prefer IPAs without a lot of age on them so that the hop character remains fresh. But you should try one occasionally and see what you think.