Using a small amount of the copyrighted work, and no more than is necessary to achieve your purpose, helps point toward your Use being Fair. Some people erroneously claim that there is a certain amount of a copyrighted work that is automatically considered Fair. The truth is that there is no magic number; it depends whether what you use can be considered the heart and core of the original work. The other factors -- particularly the first of Character and Purpose -- also influence this determination.
Click here to learn if you are protected by Fair Use