The time it takes to develop HIV antibodies in your blood after a risk exposure to HIV. If an HIV infection has occurred, then HIV antibodies will usually appear within 45 days (6 weeks) after the risk event. In some cases antibody formation may take up to 3 months to develop. The window period can be different for everybody – therefore a test for HIV antibodies may not be reliable if it’s taken before your individual antibody response has occurred, so the 3 month window period is used as a ‘catch all’ standard recommendation for repeat HIV testing. However, HIV infection (if it has occurred) is immediate (i.e. there is no HIV infection window period, only an HIV antibody window period). Post Exposure Prophylaxis (PEP) can prevent an HIV infection taking hold IF HIV antiretroviral treatment is taken within 72 hours (but optimally within 24 hours) of exposure to HIV.