Time from start of flower to finish could be variable if you are flowering a plant from seed versus flowering a mature clone.
When I say "mature clone" I mean it's ready, willing and able to flower. Not all plants people put into flower from seed are at that "ready to flower" stage. That can be a factor in finish times, since the plant started from seed needs to go thru puberty so to speak and that can add time in flower.
There are inexpensive "trichometers" you can buy to help you figure out when to harvest, after a few grows you may be able to look at a plant and tell when it's ready. Until then, you can find these at Amazon under jewelers loupe. Pretty cheap too. Good luck!
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