I started on 12/12 - though my plants preflowered 2 weeks before I flipped the light schedule.
I didn't start the lights at 12/12 when I saw preflowers 'cos the goal of not flipping to 12/12 right away was some more vegetative growth. So if you're not on 12/12 yet 'cos you want more veg growth then feed bloom nutes when you flip to 12/12 and use grow ferts now.