Normally, adult fleas and flea larvae die within a few hours after treatment. As far as how long it takes to work, the flea life cycle process can take a few weeks to a few months to complete. Eggs and pupae are not killed by FenvaStar, so basically they have to mature. When the flea pupae have completed their life cycle and become an adult flea, IF the FenvaStar is still effective, they will die. That's why in most cases REPEAT and TIMED applications of any insecticide are very important in flea control.
Reapply every 14-21 days until the fleas have stopped appearing, and then as needed afterwards. Also, be sure to treat the lawn and use something on the pets.
We have a complete page on how to kill fleas here - http://www.epestsupply.com/fleas.php