how long can mum last terminal lung cancer 17 mnths since diagnosis now had acute pulmonary edema wont go to h?

Mum age 73 very frail lost a lot weight has constitution of an ox not very mobile only comes down stairs daily on chair lift, very breathless on using commode in same room only moves for commode and too wash hands. How long can she go on like this as has no quality of life. sits on chair in living room dozing on and of throughput day same at night in bed