Like everyone said, different strains finish at different times. Do some research on your strain and see when other people are harvesting. Harvesting can also be a preference. If you like that couch lock high, letting them go a little longer is better. If you like the uplifting cerebral high, a little bit sooner is better. The real way to tell is by looking at the trichnomes with 30x magnification. The trichs are one of three colors. Clear, Cloudy, or Amber. Clear is bad, this is THCA(not pyschoactive yet), Cloudy is THC(this is good), Amber is CBD(degraded thc, this isn't bad per se, in fact may people believe that it works directly with THC to produce the feeling we all know and love. This coupound is also very medicinal). Most people like to harvest when they have mostly cloudy with some amber, and almost no clear.