I once did CPR on a dead guy for 40 minutes.

I wish we had stopped, but policy was to keep CPR going once started until a doctor or coroner says stop. His wife, an RN, had started CPR about 30 minutes before we arrived at the wilderness location. He had no pulse, no respiration and was getting colder the whole time. Me and a firehouse buddy continued CPR as we hauled him out of a rough terrain area in the woods.