You have to do 6-8 weeks of flowering, unless you want premature plants and I'm sure you don't.
Start on 12-12 light as soon as you see preflowers and consider that day 1 or day 3 of flowering considering it takes a day or 2 for preflowers to show.
I'd probably wait till I see preflowers and switch it to 12-12 & call it day 1 of flowering.