When talking about time or distance, we use the number + noun expression as part of a possessive compound noun.
If the number is one or less, or ends in a fraction: a half, a quarter, etc, the unit of measurement is in the singular and is followed by -‘s.
- It is only half-an-hour’s drive.
(a drive of half an hour)
- a week’s break.
(a break of a week)
- There is half-a-metre’s width between the two walls.
(the width of half a metre)
If the number is more than one, the unit of measurement is in the plural and is followed by an apostrophe (‘).
- It is five hours’ drive from here.
(a drive of five hours)
- We will be back in two weeks’ time.
(a time of two weeks)
- It is ten metres’ to the ground.
(ten metres to the ground)