Every fair use violation is examined on a case-by-case basis determined by the rule of thumb that fair use not diminish the “potential market for or value of the copyrighted work” (U.S. Code, Title 17, Section 107). According to the code, “There is no specific number of words, lines, or notes that may safely be taken without permission. Acknowledging the source of the copyrighted material does not substitute for obtaining permission.”
That said, fair use generally allows quotation of excerpts in the following cases:
- News reports
- In a review or criticism
- Teaching (must be nonprofit, face-to-face)
- Research (these must be “short passages”; refer to a journal’s style guide for specific guidelines)
In general you will find that organizations and publishers have their own rules based on experience (including lawsuits), the advice of lawyers (who hopefully keep up on policy changes), and codes of ethics. For example, the American Psychological Association (APA) declares that copyright holders define what they consider fair use and that the responsibility for getting permissions lies with the user (APA style, 6th ed., section 8.04, p. 233; see also sections 1.10 & 6.10).