You need to wait at least a month - the last day you took PEP is like day 1 of exposure. If you had a high-risk exposure (e.g., unprotected anal or vaginal sex with someone who is HIV+), you can ask for an RNA PCR test. Depending on how health care is paid where you live, they may not give you this test unless your exposure was high-risk, because it's very expensive compared to the 4th generation test.