how often should a cat takes tapeworm pill?

Most tape worm medication should be administered in relationship to the needs of the cat. If they are outside a lot, or are eating mice while inside, or most likely scenario, exposed to fleas, tapeworming should probably be done every few months - at least twice a year and possibly more if they are heavily infested. Usually it's best to give the cat the drontil (or other tape medicine) and then follow up in 2 weeks with a second dose to be sure and get the eggs too. So basically each time you give them a worm pill, follow up with a second in 2 weeks. If your cat has been given medication for tapes and you feel they are cleaned out - and will not be re-exposed to fleas,mice or whatever, then you can relax a bit on the whole tape worming. Don't forget to worm for roundworms and the other worms too!!

On requirement: worm segments show up in the cat's feces. Tapeworm treatment should be done by a vet; it is not suitable for home use.