If that is a photoperiod plant you can switch to 12/12 when you are ready. Anticipate the plants will double in size after switch.
Calculate your available grow area. Measure from the top of the soil to the recommended distance from the lamp to determine the maximum height you can allow your plant to grow. Flip on or before 1/2 that measurement.
I have a 42in/106cm maximum grow area. I typically switch when my plants reach about 18in/45cm in height. I just flipped my AK-48 grow the other day and expect to be in full flower stage by March 1.