According to the Doctor, Amy was replaced with her ganger before she went to America, though he was uncertain how long before. Since the real Amy was already pregnant with a child (Melody Pond, a.k.a. River Song) conceived on her wedding night, she was replaced after "The Big Bang" but before "The Impossible Astronaut". The ganger was destroyed at the end of "The Almost People", just as the real Amy was going into labour. Accordingly, the Amy we saw was a ganger in the episodes from "The Impossible Astronaut" to "The Almost People", inclusive, which is six episodes.
There's no reason to think that Amy was a ganger in "A Christmas Carol" but that wasn't completely ruled out, either.