I think scope is needed for science not to tell, when the plant is ripe.
As many people have said, plant is ready when it looks ready. And what does mean "ready"? Well, someone will look on trichs, hair, or will count time, but main thing is the smoke - how will it taste, how will it smell, what efect will it be and how long it will last. And everybody likes something different! There is no constant "ready". By experimenting You will find the way You like it the most - and than it will be ready.
P.S Sorry, if I didnt write it correct (my english is bad) or someone disagrees with me, I dont say others opinion isnt right. This is how I see it and wanted to share with it.