The most accurate way to determine how mature and ready for harvest they are you must look at the trichomes. The little sparkles. You need to magnify either with a microscope, phone camera or what's best is a jewellers loupe. They are only £5/ ish.
The heads of these trichomes will likely be one of these colours. First they are clear then they go cloudy and finally amber. Peak potency is about when 80% are cloudy . They longer you leave them and the more amber they are will produce a more sedating buzz. Hope that helps