Considering the age and the stage of cancer, it is some time very difficult to operate in some cases of cancer. Also pancreatic cancer are very difficult to operate. In last stage of cancer only palliative care
is given to have a pain free survival and have a comfortable life. Many times operation is also not done because this is not going to prolong the life time and only increase the problem of undergoing heavy surgery.
Now without knowing the stage and metastatic status of cancer it is difficult to give a time frame of the survival. I think you should talk to the doctor who is taking care of your father regarding this.
Hope for the best and help your father to have a comfortable remaining life.