In this link it says every 3 months, but there is another type of vaccine that doesn't need to be repeated in this article from University of Florida:
Newcastle disease
Chickens and turkeys can be immunized against Newcastle disease. Low-virulence live-virus vaccines are administered by a variety of routes such as drinking water, intraocular (eye drops), intranasal (nose drops), spray). Killed-virus oil emulsion vaccines are administered to pullets intramuscularly or subcutaneously as a final vaccine prior to the onset of egg production.
Chicks are often vaccinated at the hatchery against Newcastle disease and infectious bronchitis with a combination vaccine. Day-old poultry vaccinated for Newcastle disease can not be shipped through the mail.
The combination Newcastle-Infectious Bronchitis vaccine can also be given at 10-35 days. The vaccine can be administered via the drinking water, intraocular route or intranasal route. For breeder and layer flocks the vaccine needs to be repeated at 3-month intervals to maintain protective immunity. Alternatively, an inactivated vaccine can be given at the time of housing (18-20 weeks). Further vaccinations should not be required with this procedure. In breeder flocks, the high antibody level obtained by repeated vaccinations will assure transmission of a uniform parental immunity to offspring.
If you purchase pullets or mature chickens to add to your vaccinated flock, they can be vaccinated with Newcastle disease (B-1) vaccine via drinking water, intraocular or intranasal routes. The more reactive LaSota Newcastle disease vaccine is then given 4 weeks later.
Turkeys are often vaccinated against Newcastle disease at 4 weeks of age, and again when the breeders are housed.
