as long as the plant's got the nutients it should be able to handle 24/7. it's not natural sure, but it should still be able to photosynthesize constantly and not need a rest. The more light hours the better. even 12/12 is just an estimate. some strains can flower at 14 on and 10 off and you should test your own. this is good to find out, since the more light the faster it grows.