I have an IPA and a robust porter that have been in the secondary for nearly 8 months now. I accidentally fermented both of them at too warm a temperature, and got some off flavours. I tried them recently, and they have improved tremendously.
This is an extreme example, but for big beers or beers with lots of roasted grains, I give them a minimum of 2 months in the primary. They age much better in volume than in the bottle, IMHO. For the rest of my beers, I rarely even use a secondary any more, unless I am dry hopping.
One more thing -- before you bottle that IPA, have you considered dry-hopping it for a week? I would.