If seven to eight (of 10) frames are full (or nearly full), it's time to add that second brood box!
Once the second brood box (or third if you are using mediums) is 7-8 frames full, then add an excluder (if you are going to use one to keep the queen in the lower brood boxes) and honey supers as follows:
1. Starting with just foundation in your frames, add one honey super at a time as above. Kinda, a "fill as you go" guideline.
2. Starting with frames of drawn comb in your honey supers, then add all of your honey supers at one time.