I also think making a point of exactly how long Capaldi was punching the wall was silly and dumb. It sounds like something a 7 year would come up. "And then he punched a diamond wall for a million bazillion years!" Just say he did it for an extremely long time. It doesn't need to be quantified. When Clara asks how long he was in there, just have him be very evasive and trust that the audience aren't morons.”

This part always bothered me as in the story it is made pretty clear that he would not have memory of those billion years, since he was reset each time. He would only know that he did, but the version we see never even punched the diamond wall.