I don't really know Juliette........but from what I've been told, it's a gradual thing. You won't wake up one day to see a different eye colour IYKWIM? LOL I only know that because I asked a friend of mine (as I was wondering the same thing some time ago), who's daughter has brown eyes, and she told me that it was a very gradual transition for her.

Jacob's eyes are most definitely very blue LOL!