I find that for the first 2 weeks 24/7 works great to get the frst 6-7 sets of leaves, but then starts to slow down. After these 2 weeks i put it on 18/6 and it starts speeding up agen, so as the plants get older, they DO benefit from a dark period, because even though they love as much light as you give them, they still need to feel that they are growing outside as they normally would be.