You should go by the COLOUR of the TRICHOMES....use a magnifying glass to see close up...when the trichs' have began to change colour, harvest time is imminent ...Please note that as the trichomes change from clear to opake brown..they are effectively dying..therefore the THC content is depleating...you have a choice...if you harvest early,ie before the majority of trich's have turned..the 'floral bouquet' of the product will be at its optimum! the high will be a more 'in the head' type of haze. if you allow the trich's to turn the stone will be a more dopy/knock out stone! it's your chioce...a good rule of thumb is to harvest when around 50% of the plants total trichomes have changed colour