Well it depends upon the brand of vaccine and individual response Acc. to WHO an antibody titer above 0.5 IU/ml is required to show adequate response against rabies virus To ensure this WHO guidelines clearly says to check antibody titers every 6 months for those working in labs and every 1 year for those who are at risk of exposure like vets, animal workers, animal feeders etc And if the levels fall below 0.5 then only getting a booster as prophylaxis but majority of people get an annual boosters to maintain the titer

It is based on the brand. In our country, the vaccination gives immunity for a minimum of a year to max 3 based on brand.