You can actually go 12/12 from seed if you want to. Seems most people have height restrictions or are too impatient to let the plants mature on their own and flower them at 18 inches, or maybe after a measly 4 week veg cycle. Personally I veg for at least 60-120 days, then flower if the plants don't do it naturally. Basically you can flower as soon as it breaks ground, but your output is minimal.