I don't know what the right length should be. And what data do you need to back up a particular date range? The only thing I've found (and I can't find the arti

I don't know what the right length should be. And what data do you need to back up a particular date range? The only thing I've found (and I can't find the article now) was a discussion of books on Amazon, and there was a steep drop off of copyrighted books older than 25 years (I think it was they weren't even offered for sale, not that they weren't being purchased)