Post op vasectomy how many months to use alternative Contraceptive?

Post op vasectomy how many months to use alternative Contraceptive?

Back up contraception should be used for three months after vasectomy cz sperms that r already present in the ductal system would be ejaculated for the next 15 -20 ejaculates.After Three months male partner is called and semen analysis is done,if azospermia present than back up contraception is stopped

Thank you