Time is of the essence.
Based on our current knowledge, a close contact is someone who was within 6 feet of an infected person for at least 15 minutes starting from 48 hours before illness onset until the time the patient is isolated. They should stay home, maintain social distancing, and self-monitor until 14 days from the last date of exposure.
https://www.cdc.gov/coronavirus/2019-ncov/php/principles-contact-tracing.html