The law does not set an exact limit on the amount you can use. I can guarantee you that copy/pasting an entire article or a chapter from a book (both of which I have seen here at Wealthy Affiliate) would not be ruled as fair use were the courts to get involved.

The more you use, the less likely it is that you are within the fair use boundaries. Although it is usually evaluated against the overall length of the original, it's still tricky. It's not always obvious that the book chapter was also used in its entirety by the author for promotional purposes, and would therefore also be ruled as the entirety of the work.

Quality is also evaluated when determining fair use for this element of the law. If a short movie clip encompasses all the best part of the movie, or if the quote you're using from a news article is its unique, journalistic 'scoop' you will not be able to claim fair use.

However, if the entire work is used in a parody or some other derivative work that requires the whole of the original in order to be understood, then that would become an acceptable fair use.

This gets the most tricky with photographs or artwork. Recent court rulings have found that thumbnail images of low-quality resolution can be utilized under fair use as long as the other three elements are also clearly fair use as well.