We use these tabs, and they work right away. Within a few hours, the worms are gone. We've never needed to re-dose the cats, as long as we use the recommended dosage (1 tab for 9 lbs and under, or 1 1/2 for over 9 lbs). Of course, you also need to use a good flea medicine afterward or else they will eat another flea and get worms again.
To see this deal, click here.