The most common advice I have read, is that you should use 18 hours light per day during the veg phase, and change to 12 hours light per day during flowering. That's what I'm trying right now on my first attempt. It seems to work fine. The idea is that the plant think it's summer when it vegs, and then the light period is shorter, the plant will think it's fall, so it will begin flowering before the winter. That way you make the choice about when the plant will start to flower. You could change to shorter light periods after about a month or so.