Only photoperiods need 12/12 to flower, the whole point of autoflower is that they will just flower without you doing anything - therefore you cannot plan how long you want to veg them, they will just flip when they decide so, usually after 4-5 weeks.
Ideally you should just stick to 18/6 or 20/4 for the whole grow. Using 12/12 has no benefit to it with autos, instead you are just depriving them from light and therefore growth, and your yields decrease as their life cycle is pre-determined.