There are a few things that are uncommon in your father's case. First, minimal change disease developing at 82 is unusual. Also, it is very uncommon for minimal change disease to require dialysis.
If your father has not had a renal biopsy this would be good to discuss with his doctor to be sure of what is going on. The need for dialysis makes me wonder if his kidney disease has progressed beyond minimal change disease.
Follow up with a nephrologist is crucial. Unfortunately, given the uncertainties of his case I am unable to say how long I think he will need dialysis for.