I usually start cloning when I see good branching off the main stem, and by that I mean branches with more than four or five internodes. If you wait more you are going to be able to take branches of the branches of the branches haha, I dont know if I said it right, but thats going to help you be able to take more clippings, and also, helps those secondary branches keep on growing without having to cut down the whole secondary branch, since youre cutting auxiliary branches from it.