Regardless of the part of the body first affected by the disease, muscle weakness and atrophy spread to other parts of the body as the disease progresses. Individuals have increasing problems with moving, swallowing, and speaking or forming words. Eventually people with ALS will not be able to stand or walk, get in or out of bed on their own, or use their hands and arms. In later stages of the disease, individuals have difficulty breathing as the muscles of the respiratory system weaken. Although ventilation support can ease problems with breathing and prolong survival, it does not affect the progression of ALS. Most people with ALS die from respiratory failure, usually within 3 to 5 years from the onset of symptoms. However, about 10 percent of those individuals with ALS survive for 10 or more years. Here I have given general idea on how the ALS condition progress and how much person can live once it is diagnosed. In your father case as per your explanation I understand he is already in a very weak condition. And not able to walk and move around means this will do cardio respiratory reconditioning and will make it further weak. As well the ALS it self will affect the respiratory muscles too. So at this stage till the time you keep him on ventilator and respiratory support as well patient maintain willing ness to live he will be able to live but this will not be for long time. Take care.