I always thought Faith's conception was at the end of "Outlander" in the monastery the night Claire brings Jamie back from near death. They stayed in the monastery for a few weeks for him to completely heal. So based on that time frame and this passage on the last page, "The move above was a Christmas moon, so large as almost to fill the empty window. It seemed no wonder that the tides of sea and woman should be subject to the pull of that stately orb, so close and so commanding. But my own tides moved no longer to that chaste and sterile summons, and the knowledge of my freedom raced like danger through my blood. "I have a gift for you too," I said suddenly to Jamie. He turned toward me and his hand slid, large and sure, over the plane of my still-flat stomach"