The four factors of fair use must be considered together when determining if your use of something is fair. There are no exact guidelines about what constitutes fair use, but the following may help:
Factor 1: Purpose and character of the use
If the work is being used for eductional or personal reasons, it's more likely to fall into the "fair use" category. Commercial use will not likely be considered fair.
If the work is being used for parody, commentary, or criticism, it may be fair use. In other words, if the work is being used to create something new or to add value to the work, it's more likely to be granted fair use status.
Factor 2: Nature of the work
Use of works that are facts (like biographies) is more likely to be considered fair use than those that are imaginative (like novels).
If the work you want to use is already published, your use will more likely be considered fair than if you were using something that's unpublished.
Factor 3: Amount of the copyrighted work used
Are you using a small or large amount of the work?
Copyright law does not outline an exact number of words/lines/paragraphs that constitute fair use, but, generally, the smaller the amount the more likely your use will be considered fair. The nature of the excerpt used is also taken into consideration; use of only the most signficant part of a work may be harder to defend.
Factor 4: Effect of the use upon the market
Since copyright law is meant in part to reimburse creators for their work, your use should not cause the copyright owner to lose income. If your use produces limited copies and does not compete with sales of the orignal, it's more likely fair use.