You get 5 GB storage for uploaded stuff (does not count stuff bought from Amazon). After you've exceeded your storage limit for 180 days, Amazon will delete stuff, newest first, until you are back to your level of storage.
https://www.amazon.com/gp/help/custo...1514462&sr=1-1
If all you are uploading are ebooks, chances are you will never reach capacity. Good place to put unread books from other sources. Then you remember that you bought it already somewhere else.