If the breeder recommends 8 weeks of flowering then you start counting when the flowers begin to form.
Or you wait around 10-11 weeks of 12/12. But check plants for ripeness after around week 9 of 12/12.
Flowering times are approximates based on breeders finding early harvesting phenos and late harvesting phenos. Usually they quote the earliest possible harvest time based on a phenotype that finished the quickest in their trials.