You can watch the trichomes and harvest just before they get where you want them. You can snip off a small piece of sample bud, quick-dry it in the microwave and smoke it to see how the high is progressing. You can search for posts from people who have grown that exact strain and either read about or ask them about how long it should flower for at given sizes.
Buy a decent 30x microscope to see the trichromes.. Clear trichromes = not ready yet. All foggy white = 100% headhigh All amber/yellow = 100% bodyhigh 50/50 Amber/white = 50% bodyhigh 50% headhigh
Haha. I saw u just ordered Ur seeds. Don't think that far ahead or Ur weed wont grow. Just like a watched pot never boils.... Start with sprouting and healthy vegging before u get into harvest thinking. Words from smokey
Get a pocket microscope ($12 at Radio Shack) or else a jeweler's loupe (cheap on eBay), look at the trichs under 25x-75x magnification. That's the only reliable way to know when the plant is ready.