Auto flowers can be grown using any photo period. I use 18/6 The lower amount of “ON” lamp hours makes roots grow more vigorously. Some growers like to use 12/12. This allows for maximum root growth and the plant finishes much faster. 70-90 days
As far as finish time. I always monitor my harvest time by viewing the trichomes for maturity.
A longer photo period such as I use 18/6 tends to lengthen the wntire grow and produces great results in 90-110 days.