A visitor to the site writes, “How many lines of printed text must be covered before a quotation becomes long enough to be a block quotation. I used to know, but I can’t remember, and I can’t find it anywhere!”
According to Purdue University’s Online Writing Lab, the MLA guideline for formatting quotations is as follows:
Place quotations longer than four typed lines in a free-standing block of typewritten lines, and omit quotation marks. Start the quotation on a new line, indented one inch from the left margin, and maintain double-spacing.
Of course, the guideline applies mainly to academic writing in the humanities. For writing on the internet, line length is often determined by the reader’s web browser, and double-spaced paragraphs are rarely used.
If you’re writing for the web, Dr. Brian J. McFadden at Texas Tech University suggests using block style for quotations of more than 50 words. I’m not sure how he arrived at that number, but it seems reasonable enough to me.