10%.
Copy 10% of anything, free, knock yourself out. Fair use. Keep it simple for some things: one chapter of a book, maybe.
Copying the whole thing, building an billion dollar industry out of it in a way that kills the industry of the people you stole from, that's offensive, immoral, unethical, and dumb. And it should be illegal, and something that should be proscuted, and CEO's made personally liable for.
10% is fair.