When to flower is pretty much up to you. Key point is allow for double the height you have when you flower. If you start 12/12 when your plant is 12" tall then should be aprox 24" when done. I would think the concern atm would be to strengthen that stem to be to support the plant. Place a small fan to create a GENTLE breeze, just enough to wiggle plant, that will make stem stronger. What kind of lights are you using??