One source says quoting fewer than fifty words from an article or 300 from a book. "You'll need the copyright holder's permission to use any longer quotation from an article or a book."
The same book says about online sources, "There is less consensus about fair use. . . To play it safe, seek permission for any text quotation that represents more than a small portion of the whole.”
This is too complicated for a blog, and fair use isn't a settled issue. When I was in graduate school, we were told we could use 500 words without permission. (Note above they say 300.)
If you're not sure, read the copyright page of the book from which you cite. Unless it's self-published, you will see how much you may quote.
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Cecil Murphey's Writer to Writer Conference will take place January 16-18, 2015, at the Hershey Lodge in Hershey, PA, prior to the Munce Group Christian Product Expo (CPE). Faculty includes Cec, Jerry Jenkins, and Shawn and Suzanne Kuhn (SuzyQ). For more information, visit www.writertowriter.com.