How Do Doctors Determine The Prognosis For An Elderly Patient In A Coma After A Stroke, And Is Recovery Possible?

My mother was found unresponsive in her home yesterday and taken to the hospital. It looks like she fell and hit her head as there is a gash on her forehead as well as one eye is black and blue. We don’t know how long she was there before she was found. The doctors say she suffered a stroke/blood clot but don’t feel as though operating on her would be the appropriate move. She is 78 and has mild dementia and type 2 diabetes as well as a pacemaker. She is not on a ventilator and has been breathing on her own since she arrived but she is in a coma. The doctors say she may live up to 3 days. My question is how do they determine this? My maternal grandma also suffered the same situation where she had a stroke and was found unresponsive after 3 days. She had a do not resuscitate order and wasn’t expected to survive. However she did survive for several more years although her quality of life was poor. Just wondering if my mom may “recover”.