If you have your dog on flea medication such as Frontline, then you only need to administer it once per month and in most cases that is enough to eliminate a flea infestation, although it may take a few weeks for all the fleas to be completely gone.
If you are using other types of flea control, such as flea powders and such, you may find it helpful to bathe your dog frequently in order to de-flea him.
Use of flea shampoo that has insecticide that will stay on the coat and hair up your pet, keeping him protected from fleas after the bath water has dried.
You can also use a flea comb in order to remove leaves from the coat of the dog or cat, and apply inorganic flea powder such as diatomaceous earth directly onto the skin of the animal.
How often you need to repeat these procedures depends on the severity of the flea infestation and if you see your dog or cat suffering from flea bites, showing symptoms such as scratching and biting at the insects.