LOL, seriously though, I only dewormed mine once but they are my first flock at the new place and there hasn't been any chickens there before. I guess it all depends....tape worms are passed not from chicken to chicken but through an intermediate host like a snail or slug and they are relatively uncommon. Roundworms can be passed through feces so you may have to repeat at 3 to 4 week intervals as chickens will reinfest themselves through their eating habits. Nothing beats good house keeping though. Winter takes care of a lot of these pests in the environment but the parasite eggs can still be quite tough and survive in the soil.