12/12 from seed is ok.
i have one going as an experiment . took about 3 weeks to show sex its now 10days away from harvest with maybe an ounce on her 14inches tall under 600w hps.
if you allow a plant to begin alternating nodes it is usually mature enough to flower.
if you flower before the alternating nodes then the plant will stall a little and it takes a little longer to show sex.
J