The flowering cycle can take from 6 up to 10 weeks depending on the strain your growing, like pterzw says harvest is dont when about 60% of the hairs have turned brown, but that is only aproximately, to really know when to harvest you will have to buy a microscope with about 50x at least and look at the THC, They have to be amber/Cloudy color wait till at least 50-75% have turned amber.