Suspicious about window period

I read this forum lately and I am intersting in one thing. How can we really know what is the real window period? I don't want to scare anyone but I am really intersted how it is decided? Let say someone had unprotected sex and in that encounter contracted HIV. You say that window period is 3 months so that person tested at the 3 month mark,tested negative and normally move on with his/her life and did not take any tests for a long time. Many years later that person tested positive. How can we know when is he/she seroconverted,we or the manufacturer of the test? Or how can we know when is he/she infected?  How can we know that window period is not longer than three months? I know that Teak will say now that 3 months is conclusive but really Teak how do you know? You heard that for someone? For every one thousand infected person how many of them are tested  in the first 3 months? Not many I think

[Teak]

Take it to the anxiety forum. 3 months is conclusive.

[Rozga]

Very good explanation Teak! Thank you very much

[afraidtodie]

three months