How long would a std have to be left untreated to cause sterility?

No set time...: STDs like Chlamydia can interfere with fertility. A common way this happens is that the infection involves a fallopian tube. The inflammation causes scaring inside the tube that prevents the passage of the egg and sperm. There is no set time for this to happen. What matters is the location of the STD infection. Best wishes!

Varies: Some sexually transmitted infections would not interfere with fertility while others could make you effectively sterile with your initial infection.Chlamydia and gonorrhea can both scar up your tubes so bad nothing will get through them but you could have syphilis for decades and pop a baby out every year.