"A few times a year, at a guess? Really serious ones are less frequent, and I'm sure a handful of demons getting together in a pub somewhere and making idle boasts about their capacity to bring about hell on earth is practically an hourly occurrence. But I would be surprised to find out it was less than once a year on average that someone actually put a plausibly workable plan into action and needed thwarting by some combination of luck, providence, active opposition, and their own stupidity."