I have had stage 4a PCA for 20 years with bony metastasis for some 7-8 years but with long history of many treatments including chemo and not sure if has done anything but I’m still alive.
There are so many variables that make it difficult to assess longevity and importantly there are many newer interventions, if not cures, and will be more such that in 20 years have changed approach to treatment.
Do your own research and stay hopeful that there is possibility to live a very long time.