Can you please advise on approx life expectancy with low pulse ox readings?

A family member aged 70 has suffered from empheseyma for approx 10 years. She has been on oxygen for approx 4 years and in the past month her pulse ox readings have ranged between 50 and 80. The Dr's say they cannot do anyhing further. We are anxious to understand how long she can survive with these low readings? She now appears to be delerious, forgetful, etc. but the Dr. is hesitant to say whether this can continue for a year or so; or, if there is very little time left. Any experiences you can share in this area would be most helpful. Thank you.