When it comes to how much one is allowed to copy from a book or a journal article, I often read about it as being 10% of the total pages number or a whole complete chapter (whichever is higher). However, how can one apply this rule if, for example, the journal article is only 10 pages? Copying just 1 page is often pointless as, unless one is quoting a definition or something, it's often not enough to convey a point or to highlight some ideas. Any insight?
Fair use is a concept that is defined in quite a fuzzy manner, at least in the US, and there is no defined "safe" amount.
The key question is why you want to copy so much from somebody else's work, and for what purpose---i.e., how it relates to the other elements of the "fair use" test. If you are copying into a scientific article that you are writing, then even a fairly short amount is likely to be quite high and hard to justify, whereas if you are making a handout for discussion in a small class you might be able to reasonably use the full work.