Personally i go from the time i see bud formation. The only way to know for sure though is to go to nearest Radio Shack and purchase one of the battery operated magnifiers. Cut a leaf out of the bud and view the tiny crystals on the leaves. (trichomes- little mushroom shaped figures on the leaf) when they are mostly cloudy this means that it is near peak rippness. some will be clear some will be cloudy and some will be amber.(the reddish color in the trichomes, which means thc levels are declining) this is the way to scientifically tell if they are ready