This is a very important question. I have heard sermons from 5 minute sermon-ettes to 1 1/2 hour lectures all under the term “sermon.” Personally, I attempt to preach between 25 and 35 minutes. I think that longer than that and people tend to only remember what you said towards the end, shorter than that and you can’t really lay down the groundwork to say something significant.
However, this is a personal thing. I have heard some strong preachers preach on a very interesting topic for an hour and have wondered where the time went. I have also heard dry ponderous presentations that felt like 2 hours when it was really 30 minutes. So I guess we have two points. The first point is to say something significant and needed and helpful. If you present something significant in an interesting way, then you will have few people disappointed in your length.
However your average preacher would do better to preach shorter sermons because it is easier for the people to process and it is easier for the preacher to stay interesting.