Back to repotting for a moment - ALL brands of LECA, like other media components, will build up minerals with time, so that may affect your thinking about timing.
How rapidly they do so will be affected by the porosity of the pellet - both size and distribution within the particle - how thoroughly you flush the pots with each watering, and your growing conditions.
As there is nothing you can do about the medium once you've purchased it, it's the other two you can control.
If you flush heavily, you obviously do a better job of "cleaning" the medium than if you minimally water, but maintaining high humidity is not only good for your plants, it helps redeuce buildup as well.
Just like the wicking from pellet to pellet, the absorption and release of nutrient solution takes time. In a humid environment, the absorbed material remains a liquid throughout the pellet, so as it leaves, the concentration of minerals stays relatively uniform throughout, including as it is taken up by the plant,
In a dry environment however, the solution at the surface might evaporate at a faster rate than the solution can move from the interior of the pellet. As that happens, the solution at the surface gets more concentrated, which forces the interior concentration to increase, and with more and more evaporation, the solids drop out of solution and can clog the pores altogether.
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