My Dad has Stage 4 untreatable prostate cancer. What should I expect?

He's 80 years old and just received this diagnosis after a biopsy and PET scan. He was told the cancer spread to much to treat. He hasn't asked his doctor how long he has, but i want to prepare and have a rough idea of the timeline so we can prepare. He has lots a bit of weight but is otherwise feeling okay currently. Are we talking months or possibly years? What is the average range for this situation?