Well, there are two phases in growing, vegitative and flowering. The vegitative (veg period) is usually 18/6 light/dark, but it is possible to run the light more than that, just your taste really. Some people leave the light on 24 thoughout veg. Some say the roots really grow during the dark, so I like some dark. I ran 20/4 for my veg.
When you switch to flower you need 12/12. And this is very important to not let light get on the plants during the dark part of flowering. The light can make them think it isn't time to flower and they can revert to veg, or turn hermaphadite (half male/ female). This will not kill the plant or something, but it deffinitely messes up the final product.
You should read up on the differnt phases.