If you’re providing a warrantee for a real B&W <u>photo</u> that has been properly processed with a sufficient rinse time of at least an hour, you should be able to give them a lifetime warrantee providing that the materials are as “archival” as you state.
If, as others say, the prints are ink jet or laser or offset, then all bets are off. There is too much variability with inks, dyes and papers to be able to count on anything lasting through next weekend.
But, I would be reluctant to provide any type of guarantee. There are enough people who, through their own mishandling or ignorance, may try to take advantage of your generosity. Inform them on it’s proper handling, but don’t bail them out if they screw up.