Hiya, I'm Josh.
From my experience with bone cancer, the life expectancy really depends on how far it has spread, and in how many places.
I don't understand why it is not medical practice to scan for bone cancer when a person is diagnosed with any type of cancer, because if they did they might be able to save a lot more people, or give them some more pain free time with their families.
My nana was diagnosed with bladder cancer last August, life expectancy of one to two years... the docters didnt do anything about it until February, at which stage she got very sick and then they picked up that it had spread to her bones - her spine, ribs, shoulders, arms, as well as lymph glands, from what i have researched this meant a few months max. It was just over two months ago that she got diagnosed with aggressive bone cancer, and is currently on her death bed, on heavy pain medication and going into a coma, expected to only live for the next 24/48 hours.
So I guess it depends on how far it has spread. If the cancer has only spread to a couple of places they may be able to do radiation treatment to shrink the tumours, which will slow down the entire process.
I hope this helps in some way.
Good luck and God Bless
Josh