However, in quantitative expressions of the following type there is possible variation:
a ten day absence [singular]
a ten-day absence [hyphen + singular]
a ten days absence [plural]
a ten days' absence [genitive plural]
With temporal nouns in the plural, the apostrophe is sometimes omitted: several weeks'/weeks vacation
Pages 325 &1333, A Comprehensive Grammar of the English Language, by Randolph Quirk et al. 1985