Subsequent to just having a vasectomy , how long does it take for spermatozoa not to be viable?

Unknown?: General advice is to take precaution against pregnancy until post-vasectomy semen exam shows no sperm on two occasions with 2-4 weeks apart. On average, some 85% of post-vasectomy patients will have no sperm inside semen after 15 ejaculations or in 2 months whichever comes first. So, follow the instructions from your treating physician until semen exam shows no sperm. Work with doctor...

Up to 6 weeks: You need 2B careful until semen analysis is devoid of sperm. That is why patients should have a post vasectomy semen analysis. There is a 0.25 - 0.5% failure rate.