Can someone with only 20% heart function survive for long?

It depends: Estimating in survival is very hard, and probably impossible over the internet. It is dependent on many variables. About 50% of patients with serious symptoms will survive about 12 months. Those patients with milder symptoms about 50% will survive 5-6 years or so. There are many treatments available for heart failure now which can improve this, but it depends on how they respond.