In the United States, most style guides that I have encountered recommend including the second hyphen in situations such as "8-foot-long bridge." Here is how some guides frame their advice. From The Associated Press Stylebook (2002):
dimensions Use figures and spell out inches, feet, yards, etc., to indicate depth, height, length, and width. Hyphenate adjectival forms before nouns.
[Relevant examples:] the 5-foot-6-inch man, the 9-by-12 rug
From The New York Times Manual of Style and Usage (1999):
long(-), (-)long ... As a suffix, long forms a solid compound when it attaches to a one-syllable word ending in a consonant. The compound is hyphenated if the suffix directly follows a vowel or a word of more than one syllable: daylong, decade-long, hourlong, minute-long, mile-long, monthlong, second-long, weeklong, yearlong. Do not attach -long to a plural: instead of weekslong delay, write a delay of weeks.
Shorten expressions like two-hour-long meeting. Two-hour meeting says it all.
From The Chicago Manual of Style, fifteenth edition (2003):
7.90 Overview. ... number, spelled out, + noun: a hundred-meter race, a 250-page book, a fifty-year project, a three-inch-high statuette, it's three inches high, five- to ten-minute intervals. (Hyphenated before a noun, otherwise open. Note the space after the first number in the last example.)
From Bryan Garner, Garner's Modern American Usage (2003):
PHRASAL ADJECTIVES. A. General Rule. When a phrase functions as an adjective preceding the noun it modifies—an increasingly frequent phenomenon in 2oth- and 21st-century English—the phrase should ordinarily be hyphenated. Hence, the soup is burning hot becomes the burning-hot soup; the child is six years old becomes the six-year-old child. Most professional writers know this; most nonprofessionals don't.
PUNCTUATION. ... J. Hyphen [-]. ... Here's the rule: if two or more consecutive words make sense only when understood together as an adjective modifying a noun that follows, those words (excluding the noun) should be hyphenated.
Since we're talking about punctuation style here, there is no single objectively correct way to handle the hyphenation of compound adjectives; but as a matter of convention, the style guideline that yields "8-foot-long bridge" is considerably more firmly established in U.S. publishing than the one that yields " 8-foot long bridge."