If the guy had undergone vasectomy, how long does he have before reversal would be useless?

No set time. There is no set time and there are reports of reversing many years after. Most people say after 15 years the chances are much lower of success. We know it gets worse the farther out you are.
Ask a urologist. I have heard that some vasectomies can be successfully reversed up to seven years after it is done. I would ask the doctor as those who perform the procedure can give you the best information.