Only harvest once your trichomes (the little mushrooms looking crystal on bud) is starting to turn amber in which thc is degrading, 20%amber in your trichomes is a good time to harvest, and i mean 20% of all you r trichomes on your bud, doesnt need to be dxactnjust when most of them are milky white.
you'll need a microscope of about 40x to see them.