I think a 14-16 light on and u could be ok least from what I've seen and my buddys. I have found auto or photo which ever way ur wanting to move them more light or less than do a full 24 force in that direction, say u want to do 24 light and it running a 18/6 I would do a full change in one day before light comes on to run 18/6 again change it to 24 and let it rip like I just did for my girls now flower is here I'm gonna try the full 24/0 schedule and see,
But if changing a photo to more dark than give it a full 36-48 dark period before the switch to really set flower in play.
I've tried the 30min to 1 hour increments every day on autos and found I could just do it in one day and get them less stressed and less loss time to growing my all tgis brought to you by my opinion and experience.