Hello,
I am sorry to hear about your mothers heart conditions. These are always very difficult decisions.
It is not easy to put numbers on survival, especially without seeing the patient. In general, once people start having heart failure symptoms related to aortic stenosis, a range of 2-6 years is reasonable. That does not mean she might not live 10 years or 2 months, it depends on many factors and is difficult to even speculate.
Also what other symptoms should we be aware that would indicate worsening of her disease?
The triad for aortic stenosis is heart failure, chest pain, and passing out. As the mitral valve narrowing progresses, she may develop more shortness of breath and possibly more hospitalizations for heart failure.
If your mom is 86 and has decided she doesn't want surgery, then she probably understands that her long term prognosis is not good. If she understands, try to support her decision and not focus on her being sick. I know it is hard, very hard, but try to focus on living for the now and not worrying about the future.
Good luck. Thanks for posting.