It would depend on how long her kidneys had been failing before they found it and it also depends on if she qualifies for dialysis or not. I assume that if she has many other illnesses, the dialysis isn't an option and neither is a transplant.
I am sorry for what is happening. It may be a few months or may be a few weeks. My advice is to spend as much time as you can with her and cherish the time you have left. Keep her spirits up and keep your's up too. Be there for her and your family as she is going through this.
Did the doctor tell you how much function she has remaining in her kidneys? That would be a good way to at least estimate how much time she has left. If they have completely failed and aren't working at all, then I wouldn't expect much time. However if she still has some function left, then it may take a little longer.