Depends on how you want your finished product. A couple weeks will give you a few very small buds, but usually isn't worth it to try and smoke, since there hasn't been enough time given to develop and mature he trichomes. I'd recommend (and I realize this is entirely strain-dependent) letting your 12/12 cycle continue for at least a month. After that, you should see some decent sized buds. From there on, you can harvest depending on what type of high you're looking for: early with white trichomes = head high, later with dark trichomes = body stone. Hope that helps!
Buds will start forming in the first few weeks but, for the plant to fully mature its takes 7-9 weeks.