Per this NGram, the noun phrase ten men's strength has effectively no currency today compared to the strength of ten men.
But when a builder is estimating the price for your job, it's always That'll be ten hours' work, and never That'll be the work of ten hours.
Is this marked difference in preferred usage something to do with the fact that an "amount of work" is routinely and naturally "quantified" as a multiple of a contextually-appropriate time-span? The preference seems to still apply even when the "multiple" is one - it's always That's a weeks work and He's got the strength of an ox, never That's the work of a week or He's got an ox's strength.
I feel there must be some general principle here rather than just idiomatically established usage, but when considering the ELL question “The stubble of several nights” vs. “several nights' stubble” I realised that although there are many contexts where of and the Saxon genitive (possessive apostrophe) are equivalent and interchangeable, there are also many contexts where the preference for one or the other is very strong (we all know what we say, even if we can't say why).