Can be anytime you want. Some people switch to 12/12 when the plants are 3ft in height, some when it's 1ft in height. I have flowered a plant as small as 5inches when I switched to 12/12.

It all depends on your height restrictions, and if time is a factor or not, but of course, shorter veg. time means less yeild, so it's all up to the growers personal prefrences.