I use either 24/0 or 18/6 during veg and 12/12 ,of course, during flowering.
But what you should be doing would depend on what you got going and what you are tryng to accomplish just like MellowHaze said.
As far as the 24 hours on light scheme tough, IMO it does seem to help plants grow faster and with tighter internode spacing, meaning the plant doesnt stretch as much. At 18/6 I don't get much of a difference in growth and node spacing, atleast with the strains I have and it helps me to save electricity.
Your post was kind of vague though, tell us more about what strain you are growing and what you would like to accomplish and I'm sure you could get some more advice specific to what you are doing.