When you have 7 frames filled you can add another box. Feed until the bees won't take it or when you add a honey super. Generally the same rule applies here: when 7 or 8 frames are filled, add the honey super and stop feeding, if you haven't stopped feeding already.
I hived a swarm the last week of April and have already added a second brood box. I fed the bees 1:1 sugar syrup and when I checked them a little over a week later, they had filled out or was working on 9 of the 10 frames.
I find it speeds things up moving into the new box if when you add a brood box move a frame or 2 of brood into the middle of it, also when you add a honey super take a frame of the food from the 2nd box and move it up, outside frame in the 2nd box are almost always honey.