It will depend on many factors : Cause of the low performance, age, other diseases...
However, with today´s medicines and devices it is possible to live many years. And if things get really bad there is always the possibility of ventricular assisted devices and transplant.
Jesus
As jrbon mentioned, lots of factors determine this. Taking good medications, excercise and diet as well as avoiding smoking, excessive alcohol can raise the heart function rate.
In my case, I was able to push my heart function rate to close to normal from a heart failure rate, so I know it's possible.
Keep us informed.