First, the photo is too small to see any evidence of sex. When you send pictures in that you want feedback on, get as close as the camera focus will allow.
Jorge Cervantes, who has written about two dozen books on the subject, says you should wait until you have early evidence of sex before switching to 12/12 lighting. There are pictures on line of what to look for (sorry, no links- I'll try and find some and post them for you, but just Google "male female marijuana").
Cervantes says that switching the light cycle too soon can put stress on the plants and cause some to become hermaphrodites, which is a drag. If you are really anxious to find out the sex, Cover a branch halfway down with a dark cloth for a few hours a day so that it receives only 12 hours of light each day. Within 2 weeks you should have clear evidence one way of the other.