A distinction should be made between 2 functions of the balls:

The direct fertilizing function of the clay and of the NPK fertilizer it contains should last between 3 and 6 months, depending on the exact positioning of the balls (depth in the substrate, granularity of the substrate).

After this time, large solitary plants require 1 - 2 new balls, but not more than 3 - 4 balls per 100 l pool volume.

The improvement in a substrate generally consisting just of quartz gravel or sand with clay to a substrate which is much better for plants is retained even after this time.
Clay minerals (layered alumosilicates) have a similar function to ion exchangers and buffer various fertilizer substances, i.e. in the case of a surplus they absorb them and release them again as required and thus lead to a long-term improvement in the function of the soil.