The current recommendations of the leaders of the veterinary field in the US are for the first rabies shot by 4 months, and a booster no more than one year later, and after that, boosters every three years.
This is from the 2004 CDC compendium on animal rabies prevention and control:
.....Vaccines used in state and local rabies control programs should have a 3-year duration of immunity. This constitutes the most effective method of increasing the proportion of immunized dogs and cats in any population (3). No laboratory or epidemiologic data exist to support the annual or biennial administration of 3-year vaccines following the initial series.
from:
http://www.cdc.gov/MMWR/preview/mmwrhtml/rr5309a1.htm
They provide a list of vaccines by manufacturer with the manufacturer's recommendation of frequency of vaccination
see this link:
list of vaccines
Some say annual, others say triennual, so different vets may have only annual vaccines in stock at their offices, that may be why they recommend them to patients. But the state of the art most advanced scientific thinking among leaders of the veterinary profession now recommends triennual vaccination after the first booster which is one year following the initial vaccination.
Vaccination researcher, Christie Keith reports that the CDC has said, based on a 1988 data published in a 1991 study, that there has never been a reported case of rabies in a dog or cat who has had an initial vaccination and one annual booster, regardless of subsequent vaccination history, or lack of.
http://pets.groups.yahoo.com/group/BeyondVaccination/message/14257
It's true that vaccinations can have effects on dogs that undermine their health, in both acute and/or chronic ways, and that is why the profession is devoting so much effort to studying the matter and changing their position to one in which fewer vaccinations are recommended. Their position is that, because of the health risks, only as many vaccinations as are needed should be given, and no more. They should not be given casually or in disregard of the clinical and scientific evidence. Their position in favor of triennual rather than annual rabies vaccination is based on evidence that has mounted over years and become persuasive, as to the duration of immunity of rabies vaccination, evidence persuading them that the duration of immunity is certainly longer than one year, and long enough so that a three year interval can be confidently recommended as safe.