PCR tests remain the gold standard for detecting an active COVID-19 infection. 24-48 hours. The tests have accurately detected COVID-19 cases since the pandemic began. Highly trained clinical professionals are skilled at correctly interpreting PCR test results and notices like this one from the WHO.  (World Health Organization) 

Antigen test (frequently referred to as a rapid test). This test detects protein fragments specific to the Coronavirus. It can be done in a clinic, doctor's office, or hospital. Turnaround time for results is usually very quick and in some cases, results can be reported within 15 minutes.