Flea treatment for cats typically starts working within a few hours depending on the type of medication used. Oral flea medications begin killing fleas within about 30 minutes to 4 hours and can eliminate most fleas within 24 hours. Topical flea treatments usually start working within 6-12 hours and can achieve significant flea elimination within 12 to 24 hours. However, complete eradication of a flea infestation may take several days or weeks because the flea life cycle (eggs, larvae, pupae) can take up to three months to break fully. Regular application and environmental control are important for lasting results.