How long does it take to prepare a sermon?

• "The question has always flummoxed me, because it is impossible to give a simple reply. Probably the best answer is 'your whole lifetime', because every sermon is, in a way, a distillation of everything one has learned hitherto; over the years. ... I think that beginners will need ten to twelve hours. 'Twelve hours' work on a sermon is a good general rule', said Bonhoeffer." (John Stott, Between Two Worlds, Eerdmans, 1982, pp 258)