Only the Flight Instructor Certificate must be renewed every 24 calendar months. The Airman Certificate (Private, Commercial, ATP) and Instrument Rating do not expire but the pilot must maintain currency (keep them current). The cost for this can vary, but for the Certificate it would consist of a minimum of an hour of ground and an hour of flight in a helicopter. The instrument rating currency is every 6 months, but this can be done in the simulator to save costs.