How much time can someone with terminal lung cancer live, after treatment is stopped?

God only knows : Depends on the kind of cancer, how aggressive it behaves, which organs it involves, how healthy (or sick) is the patient, and many other factors. The focus should be on establishing the best quality of life for whatever time they get, be it a day or a year. Hospice is perfectly suited for this situation. Sorry if it's about a loved one.