Always hyphenate fractions that are spelled out.
one-third; three-fourths
Always hyphenate whole numbers from twenty-one to ninety-nine.
Do not hyphenate noun-numeral or noun-letter compounds:
a Class 3 citation
a page 1 story
a type A personality
Do not hyphenate percentages as adjectives:
a 3 per cent increase
Do not hyphenate units of measure when they are abbreviated, even as adjectives preceding the noun.
a 6 km road
a 6-kilometre road