When a box is packed, it’s time to split, or add more space. It takes 21 days to make new baby bees so that give you some idea how long it will be before you should even consider a split. If we’re talking deeps, a deep frame has 6000+ cells. If/when most are capped brood is a good time to split since you’ll have an explosion of bees once they all emerge. Splitting before they emerge will keep them in the new box as well. (They won’t fly back to the old box).
As others have said, it’s best to wait a while. Also move a frame with eggs into the new split; or buy a mated queen for a faster buildup.