Yeah that is really difficult to judge with any accuracy, unless someone has grown out the same strain and remembers how long it took theirs to change over, and even then it wouldn't be totally accurate. I had a 7week Kush that was about 20% amber at 7 weeks to the day. I have a blueberry that is very indica that is about 20% amber at 9weeks the last time I grew it but I think will actually be done a little quicker this time.