The use of plurals!

[yamyam2385]

When writing a sentence that includes durations, for example, days weeks or months. Why and when is it appropriate to pluralise the duartion?

for example; the patient has been experiencing pain for one week.

While another sentence would pluralise the duration. E.g this gentleman has had one months history of pain.

[Soup]

Singular
The patient has been experiencing pain for one week.

Plural
The patient has been experiencing pain for weeks.

Possessive
This gentleman has had one month's history of pain.