Dry bentonite clay does not go bad. Clay comes from earth and in general does not have an expiration date. However, they might catch pick up odors from the surrounding. So, it is recommended to store them properly. Also, if you add water to the clay it might not stay good for long as with time they get exposed to bacteria and start growing microbes. Hence, it's important to check the microbial analysis of the clay before using it.
Government regulations require products to have an expiration date. Therefore, one might find an expiration date on the container of the bentonite clay. But, as mentioned above, dry natural clay lasts for years.