What is the normal life span with interstial lung disease?

Highly variable: Interstitlal lung disease (ild) is actually not just one disease. It is the result of literally dozens of problems that can cause irreversible scarring. The scarring can lead to severe difficulty breathing and eventually respiratory failure. Some forms of ild are very indolent and do not progress. Some are relentlessly progressive and lead to death within a couple years. See an expert.