No quick and easy answer exists. “There is no specific number of words, lines, or notes that may safely be taken without permission,” the Copyright Office says.[34] You have to compare the amount you want to use against the whole work; using a large proportion may be unfair. You also have to consider whether you want to use the key part of the work; doing so may be unfair. The Copyright Office does not offer advice on individual fair use cases.
[34] Id.
