well plants usually show male in 3 days of 12 /12 but fems take about a week to show any sign of flowering. Sometimes as much as 3 weeks if you haven't waited long enough in veg for the plant to mature. So just look out for white pistils near the top of the plant in about about week from switching to 12/12. The only thing you should notice now is the plants branching out at the nodes stretching a bit, getting ready to grow buds.