Thanks, Winnie. I looked it up, and it seems that if the parents are legally married and they’re both US citizens, there’s no prescribed minimum time of US residency (there is a minimum required time for “mixed marriages” — one citizen, one alien — and for out of wedlock children). Here’s what the State Department website says:
I’m not sure what it takes to “have a residence in the United States,” but I suspect that’s a pretty low hurdle.