if its outdoors,strain,light,nutrient uptake, all have an effect on the flowering period,but as a rule of thumb i always harvest when 75% of the hairs on the buds all over the plant begin to turn darker and wilt slightly, i have had some buds do this and some buds towards the stalk still growing nice white healthy hairs. Those i leave on the plant to swell and mature on their own time. P.S. this is my outdoor rule.