Common fractions are expressed with hyphenated words, whether the fraction is used as an adjective or a noun. Mixed fractions are typically expressed in numerals (see 19.1.2, Use of Numerals, Mixed Fractions).
Of those attending, nearly three-fourths were members of the association.
There was a half-second delay before the concert hall was illuminated.
We require a two-thirds majority for consensus.
In some cases, fractions can be expressed with an indefinite article preceding the denominator. Such constructions do not use the hyphen.
The test concluded after half an hour.
A quarter may be used in place of one-fourth.
A quarter of the consensus panel dissented.