Autos are USUALLY not dependent on your light cycle to transition into flowering. Most autos will begin to flower on their own, usually between weeks 3 and 6. If you are going 7 weeks or longer with out seeing signs of flowering, the ruderalis genes may be recessive and at this point you would switch to 12/12 to try and get them to flower. This situation though is very rare and usually a result of unstable genetics.
I have grow autos under 24/0, 18/6 and 14/10. 24 hrs got biggest buds but 14 hours got best yield for wattage used.