I must admit that when I posted I didn't even consider that the phrase should not take an apostrophe. Guess that's the beauty of this place.

From "The Complete Plain Words" (1973)-
<<<Whether one should use an apostrophe in such expressions as 'Ten years imprisonment' is a disputed and not very important point. [ha!] The answer seems to be that if "ten years" is regarded as a descriptive genitive...we must write "years'" ; if as an adjectival phrase there must be no apostrophe but the word must be hyphened [ - eh? you must mean hyphenated matey...] (see HYPHEN). In the singular "a year's imprisonment) "year's" can only be a descriptive genitive.>>>

Fowler (1996) don't say nuffink on it...any others?

And I still definitely won't see the movie...

jj