As long as you want it to be, or in other words as long as you keep the lights on for 24 hours/day or 18 hours day. Switch to 12/12 whenever you feel that the plant is big or mature enough for your growing environment. The plant should at least double in size after switching to 12/12. You can start flowering as early as two weeks after your seedling has sprouted, or when it has grown a fifth node.