It depends on the individual and their genetics and immune system
How Long Can I Live With Alcoholic Liver Cirrhosis?
If you have already been diagnosed, then you may well have an advanced form of the disease. People often have cirrhosis of the liver for some time before they begin to have symptoms, but once these appear you will get worse rapidly and need to seek treatment straight away. It is one of the top causes of death in western countries and is getting worse - you can find more information here.
Forever if he keeps drinking enough to fuel his body enough to not need liver function.