Hello, sorry to hear about your grandparent. I just wanted to share with you my mums story. She was diagnosed with stage 4 Lung Cancer, it was in the lymph nodes and bone. We had 10 weeks with her before she passed. Never give up hope and make the most of the time you have. 

Hi,

my grandad was also diagnosed with lung cancer which has spread to local lymph nodes (neck and chest) and he was also given 3-6 month without treatment. However, he was able to have treatment and is currently awaiting for his third round of chemo. They have said with treatment if it is successful he could live up to 2 years. 
Please don't give up home and enjoy every minute you spend with your grandma. 3-6 month is an estimate but these things can be wrong. Sending love and thoughts your way x