When does the word 'months' get an apostrophe? [duplicate]

Solution 1:

Three months pregnant is an adjectival phrase: its head is that adjective pregnant, and syntactically the modifier cannot be possessive.

Twelve months' remuneration is a noun phrase - its head is the noun remuneration and so the modifier can be a possessive. Syntactically, it doesn't have to be a possessive, but it can be. This is an area where the language is in flux.

Traditionally it is regarded as a possessive. Twelve months' remuneration can be paraphrased as a remuneration of twelve months (though this paraphrase is not idiomatic in Modern English). Months there is plural, so in the original phrase it takes the plural possessive months'.