There is an ongoing debate on this subject. Personally, I use 24/0 light, but some feel the plant needs some dark time....it's kinda like the trim/don't trim debate. My clones always seem to root in a couple weeks regardless, so I stick with the 24/0. As soon as I start to see new growth above the soil, I check the roots and transplant and start giving them some HO lighting. There's absolutely nothing wrong with some dark time imho, either, tho. I guess it's just personal preference.