What is the current life expectancy of Stage 4 Lung cancer that has metasized to the brain?

My mom is 64 and was just diagnosed with this in April. She has never smoked a day in her life. She has undergone whole brain radiation and will begin chemotherapy in a couple weeks. She has done extremely well with the radiation. I know it varies from person to person and the data I am looking at the internet is a few years old. Just wondering if anyone has any input. She lost her husband, my dad, 12 years ago at age 56 to a heart attack. I can't imagine losing my mom too!