1. Apostrophes are used in time expressions such as "3 years' insurance" (also called "temporal expressions"). In a temporal expression, the apostrophe is positioned before the "s" for single units of time and after for multiple units of time.
Examples:
Alan was given one day's notice.

(one day - apostrophe before the s)
That is the equivalent of one year's pay.

(one year - apostrophe before the s)
My car came with a years' free insurance.
[correct the example] (one year - apostrophe should be before the s)
There is six months' interest-free credit on all sofas.

(six months - apostrophe after the s)

These are both correct.
(newspaper clipping)

This
is wrong. It should be 3 months' mobile insurance.
(newspaper clipping)

This
is wrong. It should be 4 years' free credit.
(newspaper clipping)
Applicants should have at least 3 year's
experience.
[correct the example] (three
years - apostrophe should be after the s)
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DO NOT USE APOSTROPHES FOR ALL TIME EXPRESSIONS - ONLY WHEN THEY COULD REPLACE THE WORD "OF"
The following do not have any apostrophes in them:
I lived in Africa for 3 years.

She has six months left to run on her loan.

This point causes confusion amongst many. As a rule, you should only use an apostrophe in an expression where the word "of" might have been used.
six months' insurance
(six months
of insurance)
a day's leave
(a day
of leave)
She has six months' left to run on
her
loan.
(She has six months of left to run on her loan.) (This is
nonsense. It's wrong.)
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IT'S NOT ALL ABOUT TIME
The vast majority of these expressions are time expressions, but some relate to value and distance too:
10 pounds' worth of potatoes and 1 pound's worth of onions
a stone's throw away
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