The Google Quality Raters Guidelines may provide some insight into that question:
4.5 Copied Content that is NOT Spam
Some copied content is not spam. Here are some examples: lyrics, poems, proverbs, quotes, etc. This type of content has no unique or central authority.
If the page you are evaluating appears to be from a legitimate lyrics, poetry, etc. website, do not assign a Spam flag.
If you think the page exists only to make money, you should assign a Spam flag.
That last line seems like a joke. "If you think the page exists only to make money, you should assign a Spam flag." I think that 78% of the pages on the Internet are AdSense spam.