Evolving peak SARS-CoV-2 loads relative to symptom onset may influence home-test timing | CIDRAP
Median SARS-CoV-2 viral loads, as measured by polymerase chain reaction cycle threshold and antigen concentrations, rose from symptom onset,
Median SARS-CoV-2 viral loads, as measured by polymerase chain reaction cycle threshold (Ct) and antigen concentrations, rose from symptom onset, peaking on the fourth or fifth day of symptoms. Estimated rapid antigen test sensitivity was 30.0% to 60.0% on the first day, 59.2% to 74.8% on the third, and 80.0% to 93.3% on the fourth.
Plain English:
Highest viral load is ~4-5 days into symptom onset, which means you're most likely to test positive about a week after an initial infection.
That also means that if you're IC/HR/etc. (or healthy & just don't want a disease that has a high chance of turning into long-term disability) and need to isolate from someone who's likely been exposed, the "quarantine" period should be at least 4-5 days, if not up to a week.
This has been the case for several years, not new, and corroborated by virologists and epidemiologists I've spoken to about their own protocol for infection avoidance.