Sperm incrementally moves up the foot-long vas deferens with each ejaculation. After the blockage is created in the scrotal portion of each vas during a vasectomy, it takes about 20 ejaculations and two months to completely clear out all of the sperm to achieve a zero count.  During that time, a man is firing live rounds of ammo and is still very fertile, so continued contraception (such as condoms or female birth control)  is required until a semen analysis is obtained approximately 3 months after vasectomy.