Hmm... well, I do tend to try to maximize the amount of listening I get per credit, so I have lots of 20+ hour books. Stephen Fry's reading of the complete works of Sherlock Holmes is nearly 63 hours [1] and that's just a "single" audiobook (and highly recommended). Lots of Great Courses recordings are similar. Rise and Fall of the Third Reich is 57 hours, etc. Also, there's a ton of metadata like images and PDFs, though I'm not sure how much that takes up. And in fact (just checking again) I'm at only 638 books, so it was probably more like 625 books when I did this. Apparently it adds up.

1. https://www.audible.com/pd/Sherlock-Holmes-Audiobook/B06WLMW...