When the box is full of bees, ie bees on the outside of the outside frames, then you can add another box. It's not a strict rule though you could add a box before that stage, however if you add it too early the bees can move up the middle of the hive & not ever get to the outside edges properly.
If it's taking them a while to work those outside frames, you can help them along by spreading the frames a bit by moving an empty outside frame to the middle of the cluster, just make sure you don't spread the bees to thin so they can still keep everything warm.
I would add that you do NOT want them to swarm by thinking they are crowded. I would suggest taking Oldtimer's second part of his advice. When the 2 frames 2nd from the ends are drawn and being used, but the end ones are not drawn yet, move the 2 outside frames to the middle of the cluster. That will encourage more brood rearing and let the bees have a sense of having more space. And it will make sure the bees draw all of the deep frames before moving up. Add your medium then or shortly thereafter.
I'm right in the same process of trying to get more foundation drawn out during the main flow, and I can add a bit of a caution. A frame which is already being worked does seem effective at baiting the bees into working on a fresh box of foundation, but they will often overdraw existing honey comb extra wide and leave adjoining foundation bare or thinly drawn. You will learn by experience to take this into account when moving frames around.
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