Is it '12 month's remuneration', '12 months' remuneration' or '12 months remuneration'? New Oxford Style Manual says it does not get an apostrophe is not used in 'adjectival constructions' e.g. three months pregnant. Confused.
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'.
In simple terms:
"X months' + noun (or noun phrase, etc.)" requires an apostrophe as it means "[the amount of of] X months' [worth] of noun" (e.g. Three months' wages = three months [worth] of wages = the value of the wages for three months), the genitive apostrophe is required.
"X months + modifier (adjective/adverb/adjunct)", e.g. "Three months in jail" or "Three months pregnant" or "He used three months wisely", there is no apostrophe.
Thus, months in "three months of hard work" will have no apostrophe, as "of hard work" is an adjunct/adjectival phrase.
But "three months' hard work" will have an apostrophe, as "hard work" is a noun phrase.
It should be "12 months' [worth of] remuneration," because it's quasi-possessive in such constructions: "a month's pay."
In constructions indicating length of time that a condition has persisted - "He turned in the homework two months late", or "She was five months pregnant", or "We finished the project four months early" - no apostrophe would be used.
A good rule of thumb is probably: use the possessive apostrophe for nouns ("This book represents eighteen months' hard work"); use no apostrophe for adjectives ("Somehow the task force had fallen eight months behind").