Unfortunately, there is no clear answer to this question, so the decision has to be made under consideration of different risks. It is recommended to get tested after returning from a high-risk region. In specific industry settings (i.e. food industry) it even makes sense to regularly test the entire workforce. In addition, COVID-19-typical disease symptoms or contact with a confirmed COVID-19 case indicate a need for testing.
How often a test should be performed depends on the test type as well as the work and risk situation. A test for acute infection should be performed significantly more frequently than an antibody test for immunity. RT-PCR tests detect the virus on average 4-8 days after contact with the virus, so if the test is performed immediately after returning from a trip, infection is often not yet detected. In this case it is recommended to do a second test after 4-8 days to be sure. According to current research, a test frequency of 1-2 times per week is sufficient to stop chains of infection with a high probability after the first stage. In general, re-testing is useful as soon as the probability of contact with an infected person has increased, e.g. by participating in a larger event or contact with a new group of people. We are happy to advise you on a customized testing strategy that fits to your situation.