As far as I am concerned, worming your dog every three months to offer them protection against Tapeworm, Hookworm, Whipworm and Roundworm is important for the health of your dog as well as we human. We must not forget that some of these worms can infect human as well. Keeping your dog without worm will reduce the chances we human get infected with some of these worm
Well, from my own knowledge, deworming period is once a two month. If your are thinking about cost, I would recommend you to buy the dry dog food from Eakunuba (perhaps wrong spelling as well) brand. It come with a free deworming tablet. And I strongly agreed with what Koobg said. Dog have the ability to fight the worm by itself anatomy but the problem with human are more concerned.