Putting a time frame to life or death is a godly act and is beyond the per view of humans.
The fact of matter is that your father in law is in terminal stage of cancer liver
where the chances of survival are very poor.
What matters at the moment is,you give him your love and affection to the maximum of your capacity so that he passes his last days in peace and happiness.
You must give him all the treatment which is required to be given to him at this stage which includes chemotherapy
and good nutrition.