This is not true at all for plants grown outdoors. In most of the world, you will never have stable 12/12 lighting at all, you only have it for one day and it's called the Equinox, and most places rarely get 18/6. I live at latitude +40, and our day length never goes much above 15 hours. Outdoors, plants don't have a threshold like 12/12 or 18/6. They only pay attention to the solstices. If they days are getting longer they veg, if the days are getting shorter they bloom. I put out my plants when there were only 14 hours of daylight, but they vegged until the solstice because the days were getting longer, and after the solstice they bloomed even though there were, again, close to 14 hours of daylight, but they knew the days were getting shorter. 12/12 and 18/6 are terms that apply to indoor growing, not outdoor. Outdoors all you need to pay attention to is when the winter and summer solstices are, and find a strain that is already localized to your area and will be used to whatever climate and day length is normal for your area.