Hello ClaireIsabelle1; I am so sorry about your father-in-law.  I have not had cancer myself but have helped care for friends who have.    Palliative chemotherapy will stop the cancer from growing so quickly.  Without it the cancer will become terminal more quickly.    There are no certainties about how long the treatment will put off the inevitable decline; doctors will give you a mathematical average but - in my not very extensive experience - this can vary greatly from patient to patient.  I think you will know that your father in law is declining when he stops wanting to eat normally and is generally losing his hold on the things that interested him previously.   

In the meantime concentrate on things that might help.  Talk to your f-i-l about things he would like to do; remember he is still the same person you have known and loved all these years and will still take an interest in the things he has always liked, certainly for the time being.  Follow his lead.  You will see that I have attached links to MacMillan Cancer Support and Pancreatic Cancer Action which may be helpful to you.  And of course there are also people here who will share their own experiences

https://www.macmillan.org.uk/

pancreaticcanceraction.org/

I am also attaching information from this websiteabout coping with cancer: www.cancerresearchuk.org/.../coping

Don't know if this helps at all but hoping that others will tell you more useful information from their own experiences.  Annie