As a rule of thumb, indefinitely. Chemicals packed in sealed cans are certainly good for 50 years or more, as long as the can hasn't rusted through or been punctured. Chemicals now mostly come in sealed soft packs. They are potentially just as good, but they are more subject to damage. As long as the material remains a free flowing powder and not hard chunks, it is probably OK (a little crusting is OK, just not big hard chunks).