MrStinkyanswered grow question 3 years ago 18/6 or 20/4 - good breeders will generally specify the best light cycles for their plants i.e. RQS say 18/6 for most of their autos, FastBuds 18/6 or 20/4 depending on strain, although Mephisto grow guide says 24hrs a day.
Dark periods for autos are debated whether they are useful or not, some swear by 24/0 and some hate 24/0 cycles.
Definitely 18/6 for a minimum for autos though!