In case of an infestation it is very important to treat all the pets in your house at the same time and treat your house and your yard onn the same day that you treat your pets. The life cycle for fleas is between one month and six months depending on temperature and humidity.
You can still have fleas on your pets because the life cycle can be 6 months long. If you have problem after 4 months of treatment, it is better to change the product for a few months and after that you can again start using Frontline Plus. A good product is Advantage or Advantage II. Advantage II stops the life cycle of fleas just as Frontline Plus. Both are good products with monthly application.