A 78-year-old patient is completely edentulous. He has been wearing dentures for 19 years. The patient complains of poor fixation of the upper denture. Objectively: the lower third of face is shortened, the alveolar processes of both jaws aremarkedly atrophied, the palate is flat. Mucous membrane in the denture-supporting area is atrophied. How often should the dentures be remodelled or restored?