For backyard flocks kept in a clean and controlled environment, deworming may only be necessary once or twice a year. However, if the chickens have access to outdoor areas, free range, or come in contact with wild birds, deworming more frequently might be necessary, like every 3-4 months.
It's important to consult with a veterinarian or poultry expert to determine the appropriate deworming schedule for your specific flock. They can provide guidance on the best dewormers to use and the optimal timing for treatment.