I am so sorry to hear of this. Parkinson’s disease as you know is not curable and hard to even treat. I wish I could give you an exact time frame but, not even a doctor knows. Some people I have taken care of had it for 20 yr before passing away. Other people have it for 6 mon. and pass away. Not to minimize your grandfather but only to help you understand…. he is 80 years old. He is at life expectancy. He has accomplished a lot in those 80 years and will leave a great legacy. So please start focusing on all that he has done. Start collecting pictures, awards, certificates, newspaper clippings, etc. Talk to family members and ask them questions and write down all that you can remember. Look forward to making a book that can be put together in a nice hard covered journal. If he served in any foreign wars find out if he has metals.
What a great memorial book you can make about your grandfather. You have a lot of work to do in a short time.
So the key thing is to focus on what he accomplished and done and let nature take its course in how much time he has left

Your grandfather has already surpassed the average life expectancy for an American male. People with Parkinson’s live a normal lifespan. I can’t predict how much longer your grandfather will live. No one can predict that.