Assuming your not using an auto flowering strain, I've seen most people start flowering between 1 and 2' depending on strains. However, I have seen 2 week old clones go straight into flower as well. (that would be about 9-12")

It surely has alot to do with your grow method, personally I will be topping and screening, and then flowering at 1', this will give me about 8 7" cola's per plant (or so I hope). I'm only looking for big colas for personal use, I plan on giving all the undergrowth away, so I guess, really... whatever your looking for. I want more big firm colas, less popcorn, and I'll be flowering right away -

Keep in mind, I'm on my first grow, and a novice when it comes to cannabis, but I've grown plenty of fruits and veggies, a 'weed' should be pretty simple

So if it were me, and my plants where 12", I'd be flowering, but my setup might not be like yours. My question is, why are you transplanting now? From what I've read you should have transplanted around 4-5", about when the roots were filling up the bottom of your seedlings pot. Again, my grow is a little different, but I'll be putting my seedlings directly into what I plan to flower in - Not sure exactly what that is yet, you can read my grow log if you want (not thread jacking)

Best of luck to you, have you topped or fim'ed at all?