24/7 is ideal. if you need to fill more space, save money, or worry about cooler temps then 18/6 might be worth looking into...
In my opinion run 24/7 if you want short bushy plants that grow quick. if you are trying to save money and temps you might as well run gas lantern... 12 hours on and 1 hour of light during middle of dark period.
18/6 will give less stretch and quicker growth than the gas lantern method.
Gas lantern is most efficient.