Based on what we know from other related human coronaviruses, people appear to become susceptible to reinfection around 90 days after onset of infection. To date, reinfection appears to be uncommon during the initial 90 days after symptom onset of the preceding infection.
CDC
Role of viral diagnostic testing (PCR or antigen) after discontinuation of isolation or precautions:
For persons previously diagnosed with symptomatic COVID-19 who remain asymptomatic after recovery, retesting is not recommended within 3 months after the date of symptom onset for the initial COVID-19 infection.For persons who develop new symptoms consistent with COVID-19 during the 3 months after the date of initial symptom onset, if an alternative etiology cannot be identified by a provider, then the person may warrant retesting. Consultation with infectious disease or infection control experts is recommended, especially in the event symptoms develop within 14 days after close contact with an infected person.
Persons being evaluated for reinfection with SARS-CoV-2 should be isolated under recommended precautions while undergoing evaluation.
If reinfection is confirmed or remains suspected they should remain under the recommended SARS-CoV-2 isolation until they meet the criteria for discontinuation of precautions – for most persons, this would be 10 days after symptom onset and resolution of fever for at least 24 hours, without the use of fever-reducing medications, and with improvement of other symptoms.
For persons who never developed symptoms, the date of first positive viral diagnostic test (PCR or antigen) for SARS-CoV-2 RNA should be used in place of the date of symptom onset.
Role of serologic testing:
Serologic testing should not be used to establish the presence or absence of SARS-CoV-2 infection or reinfection.
CDC oct 2020