So if you want a high THC content, you can harvest them; you can tell at first glance that the plant is ripe. But if you prefer it to be more narcotic, then wait until the trichomes turn amber. I take 80% milky and the rest amber as a guideline. You can see it better with a magnifying glass or with your phone's zoom. You can also see that many hairs are brown, and that is also a sign.