Activyl will start killing any fleas on your cat within and within 12 hours 90% of the adult fleas will be dead. It will keep on killing fleas for over 4 weeks.
It will also inhibit flea development in the environment of treated pets for at least 28 days after application. This means you don't need to buy separate products to treat fleas in the environment. This is particularly useful as up to 95% of a flea population (eggs, larvae, pupae & adults) are not actually on your pet but in the house environment.