Most people infected with SARS-CoV-2 are contagious for 4–8 days.7 Specimens are generally not found to contain culture-positive (potentially contagious) virus beyond day 9 after the onset of symptoms, with most transmission occurring before day 5. This timing fits with the observed patterns of virus transmission (usually 2 days before to 5 days after symptom onset), which led public health agencies to recommend a 10-day isolation period. The short window of transmissibility contrasts with a median 22–33 days of PCR positivity (longer with severe infections and somewhat shorter among asymptomatic individuals). This suggests that 50–75% of the time an individual is PCR positive, they are likely to be post-infectious.
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00425-6/fulltext