How long does it take for muscle to atrophy if you are bedridden?

Age and disease. It depends on the age of the patient and the underlying condition. Elderly are affected more severely. As a rough guide an elderly person loses about 10% of muscle function per day of immobility. If there are other factors, e.g., fever, nutritional impairment, trauma, and some medications, e.g., steroids, the muscle atrophy is worsened.