Products containing Fipronil will start to kick in within a few hours of application, but require time to spread over the skin's surface. This means fleas will be killed within 24 hours of coming in contact with a treated pet.
If you have an active flea outbreak it's vital you treat the whole flea lifecycle to eliminate the infestation. This includes washing pet bedding and any removable covers in the home on a hot wash and cleaning areas the pet visits in the home and the car thoroughly.
Treating the home(not your pet) with an environmental spray may be required if an active infestation is present.