Leave alone the content of Michael Curry's sermon at the Royal wedding – most people loved it, some hated it – what seems to have wound some people up is its length. A very funny tweet from a spoof Prince Charles account has the Queen peering round a curtain saying, 'Do you think Bishop Michael Curry has left yet, Philip?'

Well, come on – it was only 14 minutes. Perhaps in the age of instant, that's too long – Twitter and Instagram are doing terrible things to our attention spans. But the consensus is that he wasn't boring.

How long, though, should a sermon be?

How long is a piece of string?

I grew up with sermons that were regularly 45 minutes long and usually nearer an hour. That preacher was someone who had a great deal to say, and said it very well, generally speaking. That's not to say this semi-converted teenager listened to every word – I had various strategies for coping including counting the planks in the impressive barrel ceiling overhead, and got to know the more exciting bits of the Old Testament pretty well.

On the other hand, the closest I've ever come to walking out of a service out of sheer rage and frustration was when a young man – God's gift to the pulpit, as I'm sure he thought – spun out five minutes worth of material into a sermon 40 minutes long.

And I still recall a sermon preached in my college chapel by an utterly emotionless academic who stood up and spoke without notes for under 10 minutes on the Good Shepherd. I don't remember the details, but afterwards, none of us could speak – our heads and hearts were too full.

The truth is, it's all about church culture – and that's really dangerous. Someone might be truly gifted as an expositor, with lots to say and a gift for saying it, but be confined to an Anglican 10 minutes because 'that's what we do'. On the other hand, someone might have nothing particular to say and no great gift for saying it either, but feel obliged to fill a Pentecostal 40 minutes because, well, that's what we do. And while it's possible someone might tell the former, 'I could have listened to you for twice as long,' it's very unlikely they'd tell the latter, 'You should have stopped half an hour before you did.' We are all too nice for that.

Furthermore, isn't it interesting that it's the 'Spirit-led' churches, where worship is – in theory – free-wheeling and spontaneous, that tend to go in for long sermons? Is the Spirit ever allowed to say, 'Just preach for 10 minutes this Sunday'?

In fact, there are no rules in terms of sermon length, only guidelines. So here are four principles for preachers.

1. Don't bore your congregations. There's nothing amusing or trivial about that advice: if you make the gospel boring, you are putting their discipleship or even their souls in peril. Yes, they have a reponsibility too, to listen: but yours is the primary one, to make your message worth hearing.

2. Know your limitations. Many a preacher can speak for 15 minutes and hold a congregation, but struggle to do so for 30. Perhaps that's their fault too (church culture again) but there's no point in filling time to make a point.

3. Get better at it. Go on courses. Read more. Tell stories. Connect with real life. Get fired up. This stuff matters. You don't go into the pulpit (or wherever) to 'talk through a passage' as I've heard a preacher say he was going to do; you do it to change a little bit of the world.

4. Challenge expectations. If you're a 10-minute preacher, preach for 20. Don't apologise, but tell people you're going to do it.

And if you're a 40-minute preacher, try preaching for 20 one Sunday. There's no need to warn the congregation; it will come as a pleasant surprise.

Follow Mark Woods on Twitter: @RevMarkWoods