Most people wait until it has shown sex before putting it into 12/12. Like orange mentioned you need to make sure that your plant does not get too tall. Indica dominant strains do not get a whole lot taller, sativa strains can double or triple their height.
After plants get so old (no specific time), they start showing preflowers at the nodes. This normally happens after the nodes start to alternate, but not always. Check at the nodes and see if you have something that looks like little white hairs or something that looks like little balls or football shapes. We have several threads going on now about sexing. I'll see if I can find one and link it so you can look at the pictures.