I just saw a
recent thread, closed due to similarity with this one, that asked which of these is correct:
"a 10-hour trip v. 10 hours' trip
a 10-day vacation v. 10 days' vacation
eight-hour sleep v. eight hours' sleep
a one-hour pay v. an hour's pay"
Most of the answer is already here, but I would like to add a little to it and invite discussion as appropriate. As I see it:
"A 10-hour trip" is fine. The article
a modifies "10-hour trip", a count-noun phrase. "10 hours' trip" sounds odd because
trip is a count noun.
"A 10-day vacation" is fine, and so is "10 days' vacation", because
vacation can be either count or noncount.
"Eight hours' sleep" is fine, but "eight-hour sleep" requires a determiner such as
an: "
an eight-hour sleep".
Sleep can also be either count or noncount.
"A one-hour pay" sounds odd to me, but "an hour's pay" is fine because
pay is noncount and
an = "one" modifies
hour, not
pay.
Similarly, "a few hours' pay" is valid too because "a few" modifies
hours, not
pay. And "a few weeks' wages" is also valid.