The immunity that a puppy has at birth begins to diminish sometime
between 6 and 12 weeks. Initial vaccination usually occurs at 6 weeks
of age and is repeated every 3-4 weeks until the puppy is between
3-4 months old. Thereafter, your dog will require repeat vaccination
at regular intervals for the rest of his or her life.

Above all, follow the vaccination schedule recommended by your
veterinarian - if there is too long an interval between the first vaccination
and the booster, your dog may have to undergo the series all over again.