Every strain and setup will very. I would try both 18 and 24 and see what works best for you. I run my veg room 24/0 under CFL's and have had great results. Current strain is Chronic. Happy Growin.
I didnt mean to imply running 24/0 wouldnt give great results, just in my experience having a dark period allows more efficient growth, from my understanding, when the lights are on, the plant 1 processes light into energy 2 stores energy for use later, 3 uses stored energy to grow in that exact order meaning when the lights are on it doesnt skip the step of storing energy and use it directly
that being said having a dark period allows them to eliminate one step processing light.. this means the plant does use what it has stored, but its able to use stored energy more efficiently than processing , storing, and then using it .
I have three strains on the go right now and all three seem to react better with a dark period. I too when I started this grow went on a 24/0 light schedule..( continuos perfect day ) but then I was told what I just put up here and tried it..noticed a big diffrence in growth from lights off to lights on again. just my two cents.
Happy growing !