Quarantine is the separation of a person who has been exposed to a contagious disease but does not yet have symptoms, in order to prevent the possible spread of that disease. Quarantine is usually established for the incubation period of the communicable disease, which is the span of time during which people have developed the illness after exposure. For COVID-19, the period of quarantine is 14 days from the last date of exposure, because 14 days is the longest incubation period seen for similar coronaviruses. After 14 days, the person no longer poses a risk to spreading the virus to others because they have not developed illness during the incubation period.

References:

1.https://www.cdc.gov/coronavirus/2019-ncov/faq.html#basics