Hey I run my autos 18/6 for for up to 3 weeks and then switch to 20/4 and I find it works great for them and give them 4 hours of dark to rest and build roots/buds.
Hope that helps and happy growing
Really its the growers prerogative. I run mine 24 hours, but my electric bill is only 10 bucks a month, no matter how much electricity i use. 18/6, 20/4, 16/8 Some folks swear on letting the girls have a little dark to rest