In reply to Richard.

Limiting the percentage one can take to, say, 10% (or any other percentage) would be arbitrary and would defeat entirely legitimate uses such as copying an entire article that is licensed under a Creative Commons licence or released to the public domain under CC0.

I would disagree. If content is licensed, I think the license takes precedence over general copyright, which automatically exists the moment original content is created. The way I understand it, fair use only applies to content which has not been otherwise licensed (or possibly to licenses which include fair use clauses).