I'm so sorry you and your family are facing this. About a year ago, a member of my parent's generation (age 78) went into decline with untreatable lung cancer. She was in hospice and was mostly unconscious with no life support, getting ice to suck on (for comfort, she wanted it) when she was conscious. It's hard to remember, but I think it was about 48 or 72 hours before she died -- basically her breathing just slowed and slowed and slowed and then stopped. She was also on morphine for pain relief, as I recall, and that may have shaped what she experienced (how long she lived, and how quickly her fairly rapid decline progressed).
At the time she went to hospice we were told she might live another week or two, and it was definitely less than even a week. So while it's possible the estimate given our family was generous, it also seems some people survive longer (again, this was very advanced lung cancer with a very large tumor in one lung).
Given what you describe, I hope your BIL's dad will pass quickly and comfortably, and that he and his family will be at peace with the process, as much as that is possible.
