I think Keith's perfectly right, and also morally correct in his stance.
The most you should do is state, if pushed to so do, what the paper/ink manufacturers claim for their product. Since that suggests a hundred years or so in the case of quality materials, that knowledge should be sufficient to satisfy the normal buyer of photographic prints.
In the case of big-time art, then I'd suggest that the galleries have their own ways of handling such ethical issues, and that short of having such a business relationship, you refrain from commiting yourself or your heirs to anything at all. Caveat emptor was always a realistic approach to life.