You can go straight to 12/12. There is a thread going on on icmag about it. People get around 1 - 1.5 oz using that method. The plant is going to grow and mature before it flowers regardless of the light cycle.

Personally I would just go 18/6 for a short time then flip them to 12/12 but you can flower from the get-go if you want to.

People growing sativas indoors can go 12/12 from the beginning as well to keep height and total time till harvest down.