What's the adoption period?
Realistically I'd say it's a few months. And what I mean by that is, whenever there is a new feature (say FaceTime, Siri, Apple Maps, etc), people will generally try it for a small period of time (few months) and decide if they like it or not. If they don't like it, they'll stop using it entirely. Once that happens it'll take a huge force to overcome that inertia of indifference.
For example, I really, really tried to use Siri, but it just annoyed me too frequently by long waits/down servers, so I stopped. Maybe it's improved, but I don't know. but it will take something really compelling to get me interested in it again.