It doesnt matter. Its about the DLI, i dont know how there can be so much nonsense about a topic thats so clear. No wonder we deal with more myths than facts in cannabis growing. Pathetic.
You can give that DLI in what so ever hours, but autoflowers can slightly take more light than photos probably thats were the 18 hour shit comes from. Also the nightcycle is as important as the day. They need to sleep and 12 hours are better than 6, if you have the power to give them the DLI in 12 hours.
I give mine 16hours atm, but thats due to my LEDs, dont have more power. preferably i would sink down to 14hours light a day.
All this rumors make me sick, since even I still fall for them.