Originally Posted by ApK
I am quite certain there is no hard and fast number. Amount you copy is merely another factor in determining fair use.
As I mentioned in my previous post, Google can copy an ENTIRE document for "personal use" (for indexing purposes), but they can only PUBLISH a portion of it (such as letting you view only 3 pages or so...).
How much you can COPY and how much you can DISTRIBUTE (or republish) have completely different limitations in "fair use" doctrine.
IMHO, if Google can copy an entire book for "personal use", then so can I...