It depends which vaccination they have had, and your individual circumstances. According to their formal licensing information, the eravac duration of immunity is 9 months and filavac duration of immunity is 12 months. However a 6 monthly booster has been recommended if the animals are likely to be under high stress (and hence not produce a suitable immune response) such as does used for breeding, some rescue situations, or if there is a particularly high risk of coming into contact with the virus e.g. wild rabbits in the garden. For most pet owners, the normal licensing information will be sufficient - if in doubt, ring and ask your vet what they recommend for your situation