How much space do you have and how tall do you want your finished plants to be? I flowered at 12-15 inches and I ended up with plants 2.5-3.5 feet tall. People flower at as little as 4 inches but I am guessing they yield very little.
what glockstar21 said - it depends on how much room you have. i recently switched very bushy under twelve inch plants to 12/12 and in three weeks they are now over three feet.
try bending in smal places were room is a issue ..if ya look at my avatar its a bent to the max from the get and look at that on 1 plant in a 2 gallon container my fav is DUCKFOOT