Depends on test. Roughly 5% of infants born to hep c mothers will acquire the infection. The mothers IgG antibody to hep c that would indicate prior infection will persist up to 18 months in baby so a + test would not be specific for baby prior to 18 mo. There is a specific test for hep c RNA that can be preformed at 1-2 mo. That would be specific for the baby.See red book, 28ed 2009.P358-360.
3-6 months with RNA. Mothers pass on their antibodies to babies, so you can't test for hepatitis c antibody in babies until about 18 months. You have to test for the hepatitis c virus itself, with a hcv viral load or hcv RNA test. Babies can have fluctuating virus levels and can spontaneously clear, so they need to be tested multiple times to see if they are truly infected. Specialists can help.