A "window period" is the time needed for the body to develop a immune response to a microbial agent. The body usually responds by creating antibodies. The window period for an infection should not be mistaken for incubation period which is the time needed for the symptoms to manifest after transmission has occurred.

The window period for HIV transmission is 3 months. In some people, especially those with weaker immune system or those taking anti-HIV medications or prophylaxis, viral particles might multiply in time to produce enough antibodies for detection. For these people, it might take 6 months for antibodies to produce. This is why doctors recommend that you be tested up to six months after possible exposure to the HIV virus. It is not expected that antibodies would take longer to produce.

You might want to visit a specialist in infective diseases for additional information.