AsNoriuanswered grow question a year ago Auto dont care about light duration, she will mature enough ( or stressed ) she flowers.
She will always flower, so longer veg is good sign, plus good harvest upfront.
BUT autos goes hermie too, playing with day duration in my opinion is heavy red zone. Choose one and stay.
18/6 is optimal for autos.
If to be honest i write 18/6 but by timer its always 17.5/6.5 at mines or so . I want for them to have those minimal 6 hours of rest 4sure. So i added a bit.
Happy Growing !!!