My 93yr old mother has started with dementia. how long can she live with this ilness? her general health is ok for her age

Cannot predict: Dementia does not started as the physician diagnosed it. It could be few year earlier. The duration for dementia is anywhere from 2-20 years with the average of 8-9 years. If she is otherwise healthly, then she can be survive for next 6-8 years. Otherwise if she becomes sicker, then the time frame would be shorter.

Depends : A 93 yo with mild alzheimers dementia is unlikely to die from it. However just because it is recently diagnosed does not mean she is in early stage. Consider joining a support group and accompanying her to office visits to get a better idea of where she is.