Well I would say that after you add your bees (package or frames from a nuc) and it fills 3/4 or less of the first box, I would let them stay in one box. The package bees will decrease in population until the new queens brood start to hatch in 21 days plus however long it takes her to start laying.
If you are transferring nuc frames to your box, it will take less than 21 days but I would still leave them in a single box for a couple of weeks.
If your bees take up more than 3/4 of the first box I would add a second.
Nucs or packages, add brood boxes/supers as necessary. If the first has 7 or 8 full frames add the second. If the second has 7 or 8 full, add the third. No benefit that I'm aware of in giving them more space than they need.
The 70 to 80 percent is based on decades of experience. If you give them too much to clean, heat, cool, humidity control, and guard from diseases pests and parasites they are more likely to fail. It is like paying taxes on a house you will not live in for a while. You are proud of it but it will work and worry you to death.
Jonathan ... I like to use a deep on the bottom, I think the queens lay better not having to hop up to the next level. You really only need to lift the top two when inspecting so utilizing a deep on the bottom really doesn't add to extra weight to lift. then like the other advice let them fill the bottom box then add.
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