Gunshot wound to the left lung, how to heal?

Quickly, usually: Most GSWs to the lung are treated with a tube in the chest, which expands the lung and causes the bleeding to stop. Only 15% require surgery, usually for severe bleeding. Once the tube is out and the lung expanded, the lung heals quickly, usually a week or two.

Days to weeks : The fact that you are typing this question means you are doing ok. If you were too sick to type I'd be worried. Most lung gunshots will heal within days and the chest tube can be removed. As long as there are no other hidden injuries you should do well.