auto's light cycle

so i have never grown any autos but i was thinking of maybe doing some auto ak47 or something simmilar, now my question is that if they start flowering regardless of light cycle can you grow them out under say 18/6?? is there any advantage to more light if it plays no role in their flowering, or do you need to cut them back to 12/12 for them to finish up, also is there any difference in yield if you left them to flower under 18/6 as opposed to 12/12

Id say to give them more than 6 hours of dark, so the plant will have time to use the energy that it has stored from the previous light period. Id think also that it might take longer to harvest. but im no expert and i havent grown and auto's so its just an opinion.