It depends on your goals and your available space. The longer you wait, the bigger the plant will be when you flip. And when it starts to flower it will stretch as much as a foot. Some run 12/12 always, and seedlings switching to veg will flower right away. Just pick a time, and change the lights. If you are unhappy with the yield, let them get bigger next time before flipping. If they end up burning themselves against the lights, flip them sooner next time : )At what point would you switch from veg state to flower state?