apostrophe with expressions of distance or time
I imagine everyone uses an apostrophe with expressions of distance or time when the number is one:
It’s only an hour’s drive from here.
They live a mile’s walk away.
A stone’s throw away.
It follows that an apostrophe should also be used in the plural version, as stipulated by, amongst others, The Guardian and Economist style guides:
It’s three hours’ drive from here.
They live two miles’ walk away.
I notice the apostrophe is often dropped here, so my question is this - do you think the apostrophe:
is always optional?
is only necessary in formal writing?
is always necessary?
or that there is some other grammatical explanation that makes the apostrophe unnecessary?