I have run across a movie called "12 Years a Slave" and am a bit confused with the title. The first suggestion to come to my mind is that it means 12 years of slavery, but I'm not sure.
Could you explain this expression, please?
Can it be used in another context except as a title of a movie/book/article?
And could you give some examples of its usage, then.
It does indeed mean that the protagonist suffered 12 years of slavery: he was (for) 12 years a slave, or (as it would more commonly be phrased) he was a slave for 12 years.
This construction can be used with durations or multiplications followed by a noun, and it indicates how long or how many times someone is or was something.
Other more or less well-known uses of the construction include Hercule Poirot who frequently at some point when the plot is nearing its denouement exclaims something along the lines of, “Ah, but I have been 20 times an imbécile!”.
It is most commonly used in verbless clauses used adverbially to describe the subject of the sentence:
Five times a nominee, she has finally joined the ranks of Oscar Award winners.
In these cases, the implied verbal structure is “Having been a nominee five times”, vel sim. In your case here, where there is no sentence at all, only a movie title, it is not really fair to say that there is an implied verb at all—the construction just means “a slave for 12 years”.
According to Documenting the American South the full title of the original book (published in 1853) is Twelve Years a Slave: Narrative of Solomon Northup, a Citizen of New-York, Kidnapped in Washington City in 1841, and Rescued in 1853.
A somewhat similar construction—albeit with a preposition added to the mix—is Richard Henry Dana's novel Two Years Before the Mast (1840).
In both cases the truncated form is easy to fill out to yield the full intended meaning: Twelve Years {Endured as] a Slave and Two Years [Served as a Sailor] Before the Mast [of a Brig].
As Janus Bahs Jacquet observes in his answer, the same type of verbless construction sometimes appears in everyday speech. One interesting example began as an advertisement (in 1925, according to this source) for Listerine mouthwash: "Often a bridesmaid but never a bride"; this phrase is now an aphorism (at least in the United States) in the form "Always a bridesmaid, never a bride."
