How many seconds in eternity? The Doctor spends an eternity, or 4.5bn years (which is approximately a lot of seconds), trying to break free of his prison and escape his demons in Heaven Sent, which leads him to quoting The Shepherd Boy, a fairytale from the Brothers Grimm ("lovely fellas" who are on his darts team, apparently). The story is a rather fitting one, telling the story of a mountain of pure diamond, and a bird who comes every year to sharpen its beak on it, eventually chiselling the mountain away entirely. Now that's one hell of a bird.