When is the best time to test for hiv after exposure?

If the exposure was less than 72 hours ago, see doctor or go to ER for post exposure prophylaxis. If that is not applicable, get tested by 4th gen test at six weeks after exposure. A positive result sooner is reliable but a negative one is not. For info: HIV test window periods https://www.cdc.gov/hiv/basics/hiv-testing/test-types.html

For a genuinely high risk exposure (like unprotected anal sex between men, or unprotected sex with a known infected partner), start with an RNA test for the virus at about 2 weeks, followed by antibody or antigen-antibody (4th generation) tests at around 3-4 weeks. For less risky exposures, a negative AgAb test can be done at ~4 weeks (around 98% conclusive), and at 6 weeks (100%).