The day you switch to 12/12 lighting for flower is NOT the actual date the plant starts to flower.
For around 2 weeks the plant will start an exhilarated growth spurt. This time frame from 12/12 is
considered "Stretching" during flower. depending on the size of your plant at the time of flower
start and amount of light the stretch may be 15+ " .
When breeders say 9 weeks they usually count from the end of the "Stretch" or actual start of the plant flower cycle.
So if your breeder says 9 weeks, you may have to wait 10 or 11 from 12/12 before it is ready for harvest.