We don't know.
All Moffat has told us is that the kidnapping happened some time between "A Christmas Carol" and "The Impossible Astronaut". We know it must have happened after she got pregnant during their honeymoon. That's about all we know.
If "Space" and "Time" happened during their honeymoon, before the Doctor returned them to Leadworth, it's a good bet (although not absolutely certain) that she was replaced after those two episodes. But we don't even know for sure that's when those episodes took place.