Also sometime need to add time for screw ups.
So many factors, so what takes one person 7 weeks may take another 9 weeks.
Example: The plant was locked out for 2 weeks. May need an additional 2 week of flowering.
The microscope or loupe does not lie. When you see the trichs it is time.
Some strains do not get as amber as others. So some strains may want to harvest before being amber and some you want to be amber. This is something specific to the strain and the effect you are looking for.
Earlier not as amber is sometime more of a upper high, whereas longer more amber is usually more of a couchlock effect. That has been my experience with a kush strain I have tried at various times.
Example:
guys tells me the strain takes 56 days. Let's call this guy cash cropper.
cash cropper cares about $$ and saving time not creating the best medicine. so he will state 56 days flower with 2-3 days flush is fine. he does not own a microcope or loupe. he harvest via calender, regardless of how the plants look.
So his advice may not be the best for me.
I found 63-70 days has a more potent effect and is much better medicine. MY goal is the best medicine even if it takes more time.
Hope that helps.