My first-aid instructor:

'You can only stop doing CPR, if you are exhausted,the paramedics say you can stop, or when you are in the middle of nowhere with absolutely no chance that someone can hear and help you.'

She also said: ''The only situation you don't have to begin CPR is when the head is 2 meters away from the body.
And if it's only 1.5 meters, you kick it half a meter...''