Lighting times

My plants are about a foot tall each and I was using a florescent light up until now for them. Now I am using a metal halide light and I was wondering if I should do 12 and 12 for lighting or if I should use other hours. Thanks

well, are you vegging or flowering? If you're vegging, stay with your 18/6, 20/4, 24/0 etc. If youre flowering, then yes, go 12/12.

I'm still vegging right now. Should I leave it on 24/7 or what hours should I do?

it's totally up to you. you can keep the light at more than 12 hours a day (18/6 is typical) and keep growing a larger plant. or you can go with 12/12 and your plant will start to mature, show sex and start to pack on the buds. the longer you go with 18/6 the bigger the plant and the greater the yield but beware of running out of space during flowering when the plant can double in size.