Once a month, especially during flea season. Begin one month before fleas become active. That’ll help kill existing fleas and reduce the possibility of flea eggs. Revolution's protection begins after two hours of application and lasts for up to a month. Here’s how it works, it kills heartworms and intestinal worms once it reaches your cat’s bloodstream. It acts fast after application. Then it starts attacking fleas, flea eggs, and external parasites as it reaches the skin and hair.