My ATF's hairs go orange at about 4-5 weeks. Don't go by hairs, go by trichs. My neighbor that's been growing for 20+ years always swears by the trichs, and none of his stuff has ever failed to rock my world. You'll start seeing that frost turn a bit amber when you look close on some strains. A pocket loupe would be better, if you can buy one. They run about $10USD. Honestly, only you'll know when it's going to give you what you want; and that requires a couple runs harvesting chunks of the same strain at different points until you find a trich ratio you like. My neighbor swears by a 40-40-20 ratio amber-cloudy-clear as a baseline to tweak off of though, if you want to get ballpark ideas.