It depends on the genetics. Some strains will start to flower after just a couple weeks and harvest at 1ft. Others can go months before flowering and finish 15ft.
They won't start to flower until they are mature enough, so it could take 2-3 weeks or so until they even begin to flower, then you have another 8+ weeks until they finish. There really isn't any benefit starting seeds on 12/12 except saving a lil on electricity. Or if you only have 1 light and you already have plants in flower.