MarcXLanswered grow question 5 years ago I personally never finished automatics under 9 weeks from seed to harvest. I can't explain why breeders often say it's possible in 8 weeks. If you have a fast veg stage of three weeks the plant has only 5 weeks for flowering.
Sure they can be harvested after that time but that comes with a loss of potency.
You should look at the trichomes to see when it's harvest time. You normally have a 2 week time-window for harvest. It opens when almost all trichomes turned foggy/milky and ends when you habe about 30% amber trichomes. Harvesting during that window gives you the most potency in THC, CBD and terpenes.