Fleas can be expected to live for approximately two weeks after being treated, according to the University of Kentucky Entomology. These are typically newly-hatched fleas who have not yet been affected by the treatment.
If you do not effectively treat the pet's environment, you will miss 90% or more of the developing flea population (eggs, larvae and pupae) that will stay around long after treatment. For successful flea control, the home, pet and sometimes even the yard must be assessed and treated.
If adult fleas are still being seen beyond two to four weeks after the initial treatment, a secondary treatment should be applied.