From the Chicago Manual of Style concerning 'Fair Use'. (Whether or not you need permission.)
"Fair use is use that is fair—simply that. Uses that differ in purpose from the original, and uses that transform the copied material by changing its context or the way it is perceived, will always be judged more leniently than those that merely parallel or parrot the original. For example, substantial quotation of the original is acceptable in the context of a critique but may well not be acceptable if one is simply using the first author’s words to reiterate the same argument or embellish one’s own prose. Use of any literary work in its entirety—a poem, an essay, an article from a journal—is hardly ever acceptable. Use of less than the whole will be judged by whether the second author appears to be taking a free ride on the first author’s labor. As a general rule, one should never quote more than a few contiguous paragraphs of prose or lines of poetry at a time or let the quotations, even if scattered, begin to overshadow the quoter’s own material. Quotations or graphic reproductions should not be so substantial that they substitute for, or diminish the value of, the copyright owner’s own publication. Proportion is more important than the absolute length of a quotation: quoting five hundred words from an essay of five thousand is likely to be riskier than quoting that amount from a work of fifty thousand. But an even smaller percentage can be an infringement if it constitutes the heart of the work being quoted."