Survival in patients with advanced lung cancer has been studied, comparing no active treatment (best supportive care only or BCS) with chemotherapy. They showed that chemotherapy prolongs survival (though they  did not include patients with low performance score, where the survival would be much shorter.)

But the data and survival curve below give some general idea what to expect if a patient declines any active therapy in favor of hospice type care
 
Median Survival in Months
Number Studied Chemotherapy Supportive Care Only
778 5.5 4.0
288 6.0 2.5
359 6.9 4.8
48 6.7 2.5
86 13.8 5.8
299 5.6 5.9
161 6.5 4.8
157 6.8 4.8

Journal of the National Cancer Institute, Vol. 92, No. 13, 1074-1080, July 5, 2000

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