Here in sunny Minnesota they become very brittle within a couple of months out in the elements. If there's nothing to break them into a fine powder, such as foot traffic, they will remain in one piece for years. If they are subjected to foot traffic, such as using broken clays as pavement for a walkway, they last a matter of months before they are crushed into a sandy substance that soon appears to wash away.
We did such a walkway just so a path wouldn't be so slippery with ice during one particular winter. By spring, after only light foot traffic, we needed to re-do the walkway with gravel, which has become our permanent solution. The clays we used do not say anything about biodegradability on the boxes.
