I like to run autoflowers 18/6 from start to finish. Some people run 20/4 or 24/0. I believe that the 18/6 provides the maximum amount of light with just the right amount of darkness for optimum growth. I personally would not go below 18/6. Also 18/6 more so mimics the light schedule of long summer days.
Hope that helps.
Btw plant looks healthy.
Happy growing