In my experience, IPAs are ready to drink as soon as they've carbed (that said, I've never force-carbed one before, so the earliest I've drank one from brew day is maybe 3 weeks later, but usually closer to 4 weeks later). How long it takes for them to go downhill really depends on the specific beer, but the last IPA I brewed was best at about 2-3 weeks after bottling and started going downhill about 2 months later, and the last bottle I had at about 7 months later had lost much of the hop aroma, hop flavor, and was just bitter. It was also getting relatively dark in color (it started out with a pleasant orange color, but the last one I had was a brownish-orange).