First things first, there is no ‘right’ answer to this question. Just because MLJ preached over 300 times on Romans (366 sermons to be exact) doesn’t mean that you should! And just because that trendy church you visited on holiday keep all their sermon series short, hardly means you should ape their method. In truth, there are benefits in taking both the longer and shorter route. There are also downsides to each approach.
The big plus of a longer series is that you can study a book in greater depth. Consequently, your congregation will become intimately familiar with that book of the Bible. The book in question will not be a place where your church made ‘a quick visit’, they will have ‘lived there’ for a period of time. A book studied over long period can have a profound impact on the life of a congregation.
Preaching longer series can come with its downsides too, however. First, there can develop an unfortunate ‘sameness’ to our preaching, because we are covering similar themes and applications repeatedly. I felt this happened to me when I preached a very long series on Hebrews. I was making the same application again and again. It wasn’t that Hebrews was bad, but my preaching wasn’t sufficiently good to produce a steady stream of interesting, powerful sermons.
The other potential problem of the longer series is that we only cover a narrow tract of the whole Biblical pasture. If you spend five years on Acts, your congregation will have learned zero about the Old Testament, the Gospels or the Epistles. Unless we have other outlets of teaching, this could prove problematic.
So which should you choose: long or short? I guess its a matter of judgement. At the moment I am preaching 3 sermons on Habakkuk. But I have also just reached sermon number 60 on Luke!