Can prolong varicocele cause infertility? if so how long of a period to cause infertility?

Yes: You need to have a sperm count to check for number and viability

Difficult to say.: Varicoceles are thought to raise the temperature of the testicles or cause blood to back up in the veins supplying the testicles. Although the mechanism by which varicoceles affect fertility is poorly understood, varicoceles seem to help damage or kill the sperm. they should be surgically repaired or by percutaneous embolization, as soon as the diagnosis is made, to prevent further damage.

Controversial: Not clear connection between varicocele and infertility. Many fertile men also have a varicocele. Not recommended to repair just for fertility sake.