You will see that the buds are mature when the hairs from the bud start to turn brown, wait untill ~50% of them are brown and harvest. this is one way of knowing. More experienced growers look at the thc with a magnifying glass to see if it's ready.They look at color(clear,foggy or amber) and the look of the resin gland(small mushroom like resin formation) it self. And then depending on growers preference they decide if it's time to harvest or not.