Frontline Plus is a spot on product for the prevention of fleas. The time between administration is 4 weeks. In extreme cases which involve a big flea infestation it can be applied as often as 3 weeks apart but this is to be discussed with a veterinarian as other measures have to be taken.
Please make sure you are following the manufacturers indication when applying the product. Read the leaflet enclosed. There are situations when the effectiveness of the product is reduced, like washing your dog with an emollient shampoo before the treatment or in the first two days after the treatment. There is some more information on the leaflet but I will leave you to find it.