Consider the following statement:
I have four assessments due in a weeks' time.
Here, the four assessments are due in one week. I am unsure if the apostrophe is placed correctly, i.e. is weeks' or week's more appropriate in this instance.
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An apostrophe showing possession always goes after the thing that is doing the possessing. You might not think of a week as possessing time, but that is actually what it's saying - the amount of time that is contained within a week. It often helps to turn the phrase around in your head to make it clearer:
The girl's toys: the toys belonging to the girl
The girls' toys: the toys belonging to the girls