Most people keep autoflowers at 18/6 but in all honesty, they will grow in any light - even 12/12... but they won't perform as well at that lowered light frequency. I only grow autos and when I grow them in a tent, the lights are on 100% 24/7 and they do magnificently. At the moment, I'm growing some indoors AND outdoors... they are outside in the sun during the day and then I bring them in to a tent at night where they get the rest of the 24 hours from artificial lights... UNTIL they get too tall for the tent (which some are now) and then they stay outside 24/7 and get whatever length of sunlight nature provides.
Good luck!
p.s.... the 12/12 flip that you've probably heard of is for photoperiod plants - flipping their lights to that schedule tricks them into thinking it's time to go into flower.... autoflowers don't need that trick - they do it on their own schedule, light be damned...