This is a very good Question. It is too bad that there is not a universal answer. It is highly dependent on where you preach. For example, there are some contexts where preaching longer than 20 minutes may cause people to start fidgeting. There are other contexts where the people would feel cheated if you only preached for 20 minutes. While there is no universal answer to the question, there are some principles to keep in mind.
Principle 1 – Don’t preach any longer than your material
If you material is 15 minutes. Preach those 15 minutes with strength and vigor, and then sit down. If you preach longer than your material, then you will begin rambling which is of no use to anyone.
Principle 2 – Preach the message and sit down
If you have come to preach a particular sermon, think very carefully about adding things to that point. You must support the point of the message, but if you add things, however good these things may be, you will detract from the original message God gave you to preach. Most preacher have more material than they know what to do with, but people cannot hear everything you have to offer even if they wanted to try. Find your point and preach that.
Principle 3 – Listen to the People AND the Spirit
If the people are leaving from the message, that may be a cue that you are going long. Now it is true that the Spirit may tell you to say certain things that may not be popular. We as preachers must hold to that prophetic voice, however, we also must recognize that every thing that tells us to go long may not be the Holy Ghost, but perhaps some other Spirit. Listen to the people and the Spirit to determine when it is over.
If you listen to the people and the Spirit, preach the message God gave you and only that message, and then don’t preach more than your material, you will more than likely hit a right balance. I have found that for myself, my optimal sermon is between 25 and 35 minutes, thus I have a bias towards that length of time. I normally tell preachers to preach 30 minutes unless they really have good reason to go longer. I do have to admit though that there are very strong preachers who preach for an hour.
So how long should you preach? Shoot for 30 minutes unless you don’t have enough material for 30 minutes, then preach the material you have. If you have more material than 30 minutes, then preach as long as you are on the main point and the Spirit and the people are all in agreement that you should continue.