Cloning-hours of light?

When cloning using aerponics until rooting should the flourescent's be at 24 or 18 hours light? I have always gone 18 but have gotten conflicting information. Secondly I use flourescent's but have been told I could use the 400 HPS but I thought that would be too much for the clones. Any advice? Thanks in advance.

it really does not matter, you can root your cuttings on anything from 16-24 hours of light as with veg
if you want to keep them constantly warm because the light is the only heat source maybe 24/7 would be better
i have mine on for 18-20 hours

using the hps for rooting cuttings is just a waste of electricity, cuttings only need 50 watts or so of florescent light