Once 1/2 to 3/4 of the pistils have changed to the darker color u should be good to harvest, if u wait to long the THC will degrade into CBN and CBD which will give u less of a less of a high and a more stoned/lethargic feeling. At this point u should also be feeding her plain water after u flush the soil. That way she uses up all her stored starches and sugars which will give u a smoother smoke.