Not sure how long it takes for them to totally decompose, but as they do, they leach out the higher than normal sulfur content that binds them together.
Keep in mind that most of these clays are either hit and shattered, or they break when they land un hit. Either way, over time, they cover a wide area, that is then denuded of any future growth.
...With regular Clays, they break apart the same way, but everything then grows up around them, and if the grass is cut, such as in a Skeet or Trap field, the residue is ground up, and everything still grows below it.
...Also keep in mind that with all of that additional sulfur content, you may be changing the PH of the soil, that can then cause lead to deteriorate, rather than remain inert.
...I love the concept, but it isn't there yet.