modifiers composed of a number and unit: "a five-kilometer hike." But rewrite such expressions as "a 30 X 40 cm aperture."
modifiers involving comparative and superlative forms: better-qualified candidate
In a range, if the word "from" is used, the word "to" must also appear: "from 7 to 15 min," not "from 7–15 min." "A period of 7–15 min." is O.K.
Break words that always have hyphens only at the hyphen.
The above are hard-and-fast rules. Below are highly desirable guidelines that should be sacrificed only if the alternative is an ugly layout.
Do not hyphenate modified adjective phrases: "short-lived compounds," but "very short lived compounds." Break this rule for scientific terms involving a string of modifiers if to do so would clarify which items modify other items.
Do not hyphenate more than three lines in a row.
Break compound, unhyphenated words only between the roots: thunder-storm, micro-physical. (However, remember to check Webster's Instant Word Guide first.)