What is the life expectancy of someone with Lung Cancer?

My mother was just recently diagnosed with Lung cancer, only thing is, it has metastasized to her brain and bones. We only found out 2 weeks ago after her memory had been lost. After taking Decadron her memory has been better but now she has lots of trouble walking. I need to know how long she has, so I can prepare mentally and to arrange for a funeral….Right now I am in the dark. Can someone please help!