From what I read 18/6 would be better or just plainly..it'd be good to give your plants at least 6 hours of darkness to allow the growth of roots. I read this article once that explained it basically saying that when the lights are on it's like a charger and the plants are charging up, when the lights are off it uses that energy given when the lights were on and uses that for further growth such as with roots and such. And also I think switching to 12/12 too early could cause the plants to stress to flower which could possibly promote hermies and at the same time I also heard that switching them to 12/12 too early would have no effect because they'd still just continue to vegetate until they're ready. I dunno but I'd just say don't switch them too early.