"The genitive can be found with nouns denoting duration, value or distance (= the so-called genitive of measure). There is no alternative construction with of. . . . When such a phrase begins with the indefinite article, the latter is a determiner to the noun in the genitive, not to the noun head. Since this is not possible when the genitive is a plural, phrases consisting of a followed by a plural genitive phrase and a noun head (e.g. a five minutes' talk) are not normally used. (They are occasionally found but not not generally considered 'correct'.) Instead we normally use the construction in which the noun is uninflected."
Declerck, Renaat. A Comprehensive Descriptive Grammar of English, p. 253. Kaitakusha, 1991.