What you want is alternating nodes. This tells you it's ready to be flowered. You change light to 12/12 and then you should see some tiny little spike come out. If these little spikes have a little ball on the base, chances are it's a male. You'll need magnification to tell. If it's a female and the plant is still small at that point, you can revert back to vegging. Personally, I would just wait until you can definitively tell the sex. This will usually occur after about 2 weeks of flowering. That's not a rule of thumb but just a guide.