How long does it take before an sti makes you infertile?

My best friend just found out she is lieky to have an STI and if she does have it shes had it for over a year and we were wondering how long you have to have an sti before it makes you infertile!

It depends on the disease. Some, like HPV, you can live with your whole life. It causes ovarian cancer in some women, but does not otherwise interfere with fertility.

 
I think you mean STD. It would depend on what the STD is. If she thinks she has one she should definitely seek attention via a doctor, a clinic or the local Health Service.

It might actually not be a Sexually Transmitted Disease but rather some other kind of infection.

It should definitely be attended to.