Examples and Observations:

• Style guides do not agree on the minimum length for a block quotation:

  Chicago [The Chicago Manual of Style] suggests setting off quotations that are eight lines or longer, WIT [Words Into Type] puts the cutoff at five lines, and APA [Publication Manual of the American Psychological Association] calls for setting off quotations longer than forty words. Many publishers have in-house rules that define "longer" as more than, say, six or eight lines.

  In some cases, two or more short quotations may be put in block format so that readers can easily compare them.
  (Amy Einsohn, The Copyeditor's Handbook. Univ. of California Press, 2006)

• Block Quotations, MLA Style
  Researchers in English literature usually follow the style guidelines of the Modern Language Association (MLA). The MLA Handbook for Writers of Research Papers (New York: MLA, 2009) offers this advice for creating block quotations:

  If a quotation extends to more than four lines when run into the text, set it off from your text by beginning a new line, indenting one inch from the left margin, and typing it double-spaced, without adding quotation marks. A colon generally introduces a quotation displayed in this way, though sometimes the context may require a different mark of punctuation or none at all. If you quote only a single paragraph or part of one, do not indent the first line more than the rest. A parenthetical reference for a prose quotation set off from the text follows the last line of the quotation. (94)

  One inch is equivalent to ten spaces.
• Introducing a Block Quotation
  "If you feel compelled to include a block quotation in a brief, assume that the judge will not read it. You must trick the judge into learning the content of the block quotation. You do this by summarizing the substance of the block quotation in the sentence immediately preceding it.
  
  "Thus, do not introduce a block quote: 'In Smith v. Jones, the Court held: . . ..' Rather, introduce the quote: 'In Smith v. Jones, the Court held that our client wins and the other guy loses: . . ..' By using this form, the judge will get your point even when he does not read the block quotation."
  (Mark Herrmann, The Curmudgeon's Guide to Practicing Law. ABA, 2006)
  
  "Present a prose quotation of five or more lines as a block quotation. Introduce the quotation in your own words in the text . . .. If you introduce the quotation with a complete sentence, end the sentence with a colon. If you use only an attribution phrase such as notes, claims, argues, or according to along with the author's name, end the phrase with a comma. If you weave the quotation into the syntax of your sentence, do not use any punctuation before the quotation if no punctuation would ordinarily appear there . . .."
  (Kate L. Turabian, A Manual for Writers of Research Papers, Theses, and Dissertations: Chicago Style for Students and Researchers, 7th ed., rev. by Wayne C. Booth et al. The Univ. of Chicago Press, 2007)

• A Cautionary Note
  "Too many [block quotations] may make your writing seem choppy--or suggest that you have not relied enough on your own thinking."
  (Andrea Lunsford, The St. Martin's Handbook, 2008)