Outdoor is a bit tricky, because various factors can influence the grow. Keep a good eye on the flower, they will tell you when to harvest. Trichomes will start to change, they have 3 stages: clear, milky, amber. You should invest in a cheap portable microscope or good magnifying glass. It's tough to spot the right time when you are new to our hobby.
There are plenty of guides out there, when to harvest. I just drop this one, because I trust it. You can decide for yourself, but you should know about the ups and downs of different stages of harvest time.