If you want the most yield for this grow, you want to switch to 12/12 when the plant is a little under half the height to about 6"-8" under the lights. I say 6"-8" because I found my plant in full flower could be closer to the light without burning. Normally you'd want that light at least 14"-18" away.
And you will need a much bigger pot to support the root structure of your larger plant. I don't do soil, so maybe someone with experience there can weigh in. But I think you want to transplant NOW to give your baby some time to adjust to the change.
If this is your first grow and you don't want to be too adventurous, that's fine. But if you top now for four main colas (see Uncle Ben's technique) you won't hurt the plant and could increase yield. I'm just making that suggestion because if you're investing all this time, money and effort in a grow you should have one eye on maximizing that yield!
But do what you're most comfortable with. So, if you repot your plant now, let it grow to just under half the grow height that you have (the height of the tent minus the light and some space under it), then go to 12/12. My guess is that would be in about 2-3 weeks.