We are not actually given precise details of how and when the narrator's mother turned blind, however we can infer quite a bit from the text. What is tricky about this story is the way that it uses flashbacks to narrate the content, which means that it starts off in the present, with the narrator talking about her old, blind mother, but then goes back to specific events that occurred in the past--one of them even before the narrator was born. However, it is clear that when the story begins in the present, the narrator's mother is blind, as the result of "encroaching and stubborn cataracts." However, she still maintains the same "catlike precision" that she had in her youth.
We can infer from the way that the flashbacks are narrated that this blindness has only started recently, as it would have been immensely difficult, if not impossible, for the narrator's mother to save the narrator from the burning house if she were blind. You might benefit from re-reading the story and paying particular attention to the flashbacks and how and when they are introduced to try and keep track of the chronology of this great story. This should help you to understand it better.