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 are markedly atrophied, the palate is flat. Mucous membrane in the denturesupporting area is atrophied. How often should the dentures be remodelled or restored?