PetLock Plus Flea & Lice Treatment For Cats is effective for up to three months, but for best results, apply PetLock Plus for Cats once a month. This will kill the fleas and destroy the eggs and larvae that can develop quickly into adult fleas, in those high risk re-infestation areas. Because flea eggs and pupae can survive for some time outside a host, it is important to treat your environment properly to completely eradicate these pests once they have infested your pet and home.