I have found that the plant should triple in height during the 12/12 cycle.
and remember if you are using c02
that you are accellerating the growth cycle quite a bit
so make sure you have enough room or better yet use LST low stress training or sea of green method if they are getting too big.

I have overgrown my space a few times and it really sucks to have to chop off a plants head early.

or harvest a plant early cuase you need the room.
but if your lucky it'll happen!