I've seen a similar post where two growers were going at it between 24/7 vs. 18/6 vs. 12/12 up until flowering, they were throwing facts around but honestly i can't quote any of it. what i did take from it was that it also depended on the lighting. seemed as if there was no significant difference in either of the test grows, but I would imagine if you were growing under cfls (as i do) 24/7 may work better since your giving your plant light that's less intense. If i were to do a grow with an HID. I think i would go the 18/6 route if only to save on energy and like i said they would be getting more intense light in those 18 hours. I'm no expert like i said. Just some of my own personal logic. Don't know if it makes much sense