Depends, i was looking at that light and i cant tell if your running a 4 or a 600 w hps. If its a four then id flower in a week or less. If its a six then id wait two or three weeks and really make this worth your while. Like mentioned above though, you have to take into account that those plants will probably double or more when you change to 12/12. Its called the stretch and the older they are usually the less they stretch but they always stretch. I flowered a two week old plant and it grew 5x the original size....booya.
I really hate those batwing reflectors because you cant air cool them and if your plants grow to close to them the trophy buds can get crispy from the heat. Thats your limiting factor for when to flip. How much room you have and how big of a light your packing. I hope your keeping track of your days from sprout. A typical clone is like a 14 day old sprouted plant so subtract 14 days from the germination date when your clones root when you want to figure out what to expect for height on your next generation. I usually try and take clones two weeks before i flip, veg for three months, and in that three months the other room buds and finnishes. Then two weeks before the finnish date i take my clones from my vegging plants again, and the cycle starts over.