You can always flush from 7 days to 2 weeks before harvest , so the tricky part is to determine when to harvest , in soil a long flush is suposed to be better and to bring more flavors , a flush too short will not be efficient
To me the best answer is : when the trichomes are near to be ready !
When you harvest a cannabis plant it is better to do it based on the trichomes ratio , so when you are closed from the targetted trichomes ratio you can start to flush
so here is a link to know more about that : https://www.alchimiaweb.com/blogen/harvest-marijuana-plants-trichome-ripeness/
You can also harvest thanks to the breeder recommendation ( add 1 week or 2 to what they are claiming )