I don&#x27;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&#x27;ve found (and I can&#x27;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&#x27;t even offered for sale, not that they weren&#x27;t being purchased)