My Mother Is 56 And Had A Double Bypass Approx

My mother is 56 and had a double bypass approx 9 years ago. The surgeon told us only 10 percent of her heart works and she might live another 10 years if only she stays on a strict diet. Approx 9 years later here we are, she is still alive and has had two stents put in her one main working artery. I want to ask a professional what we could project or assume for her lifespan. I know there is no exact answer but could she easily live another 20 years or is that a far fetch?