It is hard to answer a question like yours because the subject is very painful. Pancreatic cancer is a very fast spreading cancer for which there is no cure. There is no cure for most cancers and don't be fooled by claims that there is one. The people saying that are usually trying to take money from people willing to try anything to stop this terrible disease. There is nothing you or anyone else can do to "stop" pancreatic cancer. The best you can do is listen to his doctor and stay close to your grandpa. Try to be his advocate, helping him communicate his needs with his medical team to keep him as pain free as is possible. That should be the focus right now -keeping the pain tolerable.
The best person to give you an estimate of how long your grandpa has left is his doctor. The posters here are mostly not doctors or medical professionals and we don't try to act like we are. All we can do is try to share with others needing information what we have learned about cancer while researching and studying our own cancers.