No one and that includes a doctor can give a reliable estimate of life expectency!!
Sorry to hear of your mother's condition. Vascular dementia is almost the same Alzheimer's...the etioloy is different but the symptoms are almost identical, and that may help explain your mother's refusal to take any medication...its a cognitive disorder as well. It is estimated about 5 years (mean estimate) of life expectency after the onset of Alzheimer, a little longer with medication (aricept).
For your mother, the danger of faster than a hopeful, slow progression from an end stage of life would be the unwillingness to take medication to lower the blood pressure. It is estimated that about 50 percent of the cases of vascular dementia result from hypertension, or high blood pressure, and that appears to be your mother's situation as the focus is on bp..
An EKG is usually given to evaluate any heart rhythm disorders with the heart! What is the status of your mother's heart. The main concern as I read your post should be to lower the blood pressure as that can cause a fatal stroke, etc...it can happen at any time with high blood pressure and with no treatment it is difficult to estimate, and if she is unable to walk very far could indicate a severe heart problem at the end stage and that can result in irregular heartbeat and heart arrest at any time without any warning.
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No one on this forum can give a life expectancy for your mother you may find more info on the expert forum which a DR will answer your questions to the best of his abilities. you will need to probley give more info on findings though.My prayers are out for you and your family.