I would think you could tell after a couple of inches, you could probably see the hair start to coil, I would think if you had 2 + inches of new growth, the curls would start to coil around each other if you have curly hair, my hair doesn't seem have a 3rd dimension if that makes sense, it lays in the "s" shape against my head...
And I don't think everyone grows in "s"'s, the 4's grow in z's right??
I'm no expert though.