Many people incorrectly believe that bright-line numerical rules exist to determine how much copying of a work is fair use. This is a misconception. The substantiality of the portion copied also has to be considered. Copying even a couple of bars of a song may tip the scales against fair use if those bars are the heart of the work.
One persistent myth in popular copyright lore is that copying no more than a specified number of words in a literary work is automatically fair use. Some versions of this myth set the limit at 20; others, at 25, 30, or 50. None of these claims is true. A haiku poem, for example, might have as few as three words. Yet a haiku poem can be fully protected by copyright. Copying a haiku in full is not likely to be fair use. There are no simple mathematical rules you can apply to evaluate fair use. Courts apply the four-factor test to evaluate whether fair use has been made of a work or not.