A lot of growers do go by hair but they are wrong. These are the noob growers. You have to go by the trichomes. My bubba kush has all red hairs at like 35 days. Does that mean it is ready to cut down? The trichomes dont start turning brown and dying on it till like day 72 but you could cut it at 56 when it just starts to show some amber ones or wait a week or 2. Its all about when you want to do it but you cant rush. Some strains may have the hair turn color at the right time but you cannot always go off of this you will soon learn as you grow more strains.