Adding another box to your beehive will depend on several factors such as the size of the hive, the strength of the colony, the season, and the availability of nectar and pollen sources.
Typically, beekeepers add another box, also known as a super, when the current one is at least 80% full. This is a good indication that the bees have run out of space to store honey, and they need more room to continue their activities.
However, if you add a box too early, the bees may not be able to efficiently use the space and may neglect it, which can lead to problems like wax moth infestation, hive beetles, or even unwanted swarming.
In general, adding a box during the spring and summer months when nectar and pollen are abundant is a good idea as this is when colonies grow and expand the most. During the fall and winter, beekeepers may remove boxes if they feel that the colony is not strong enough to maintain them and prevent them from becoming a heat sink, or if they are concerned about winter survival.
In summary, the best time to add another box to your beehive is when the current one is at least 80% full, during the spring and summer months, and when the colony is strong enough to make use of the additional space.