Pancreatic Cancer life expectancy Without Treatment

My mother has just been diagnosed with pancreatic cancer. The tumor is 2cm and appears as though it has not spread. Am I correct in thinking that this would most likely make it around a stage 1b? Because of the risks as well as the dismal long term prognosis, she has declined whipple surgery and all other treatment. I know that no two cancers or people are the same, but was hoping to get at least some Idea of what the average life expectancy of someone with untreated pancreatic cancer at this stage tends to be.