The most famous looooong loading time I have read about on here was the guy who spent a couple of weeks working out how to load over 3000 books onto his PRS505 by arranging his ebooks in a series of folders. I think he said it took 1 1/2 hours to rebuild the database after he disconnected it from the computer
The obvious question is WHY !
I keep about 200 on my PRS 650 which takes about 30 seconds to refresh and is enough for a couple years of reading.