There are few things more disheartening as a congregant than hearing a forty minute preacher preach for fifty minutes, a thirty minute preacher preach for forty minutes, or a twenty minute preacher preach for thirty minutes.  Somehow, that last ten minutes can weaken and even destroy the impact of all that has been said in the sermon to that point.  There is no virtue in length for the sake of it.  I think I’ve heard two preachers in my entire life who could preach for an hour; and most preachers I know would be much better if they shaved at least five or ten minutes off their typical length.  Get up there, say what you’ve got to say as clearly as you can, and then sit down again.  That’s all that’s necessary.   As Luther says elsewhere in Table Talk (2643a), `I hate a long sermon, because the desire on the part of the congregation to listen is destroyed by them, and the preachers hurt themselves.’   And, as usual, Luther got it right.

Carl Trueman, “Luther on the Marks of a Good Preacher, II”