You may have just hived a package, set up a nuc or are just curious as to the “correct” time to add another box to your hive, but unfortunately I don’t have an answer for you. Let me rephrase that, I don’t have the “correct” answer. There are so many variables, and so few pitfalls, that there are many ways to decide whether your hive could use some more room, therefore, there are many “correct” answers and very few “incorrect” ones.
First lets figure out why you are adding more boxes, is it to give more room for the queen to lay, or are you hoping to have it filled with honey?
If your answer to that question was that you wanted it filled with honey, add the box. If your hive has a couple or a few brood boxes with large amounts of brood, bees and an active queen, add the box sooner rather than later, you don’t want those bees to fill in their brood area with nectar. You are unlikely, at this point, to create a situation with too much room. The bees have already established themselves in this location and can only really be encouraged to swarm with too small of an area, adding more space, in almost every case, will not encourage them to swarm, abscond or leave in any way.
If your answer to the question was that you wanted more room for the queen to lay, there are some other things to check into first. Does your existing brood area have frames full of honey separating brood frames? If so, remove them, either to the outside of that box (against the wall of the hive body) or to your shed for later use or extraction and replace them with brand new foundation or an empty drawn frame. If your brood area is solid, and unimpeded by nectar or honey, ask yourself how many frames in that box are “full”. In this case, “full” means drawn comb and either storing brood, pollen or nectar. Fully drawn but empty frames that are covered with bees can also be considered full. If your box has 6 or more full frames in a ten frame box, add more room. If your box has 5 full frames, but appears to be overrun with bees, add another box. Anything else, give them some more time before adding more space.
The only hard lined rule in this is; your hive WILL swarm when it feels cramped. So when in doubt, add another box. Colonies are only likely to completely abscond from a hive if they had JUST moved in or if something drastic happened to it, like a bear, flood or other disaster.