Wonderful article about the perfect length of sermons here (H/T Taonga).
You will never hear from my lips as an exhortation, "Sermonettes make Christianettes."
Sermons are messages, not lectures or encyclopaedic transfers of information or courses in discipleship. (Short change on the imbibing of appropriate theological knowledge and we may have 'Christianettes.' But the imbibing of that knowledge can happen in many ways apart from sermons = messages.)
Also, I suggest we distinguish usages of the word 'sermon.' There is, for instance, a place for a mid-week or Sunday evening 'Bible study' in a lecture and/or tutorial (e.g. with discussion groups) format. Such longer expositions of Scripture (and/or theological subjects, anchored into the Bible) are important for Christian formation (as they have been in my own life). But I suggest it is a mistake, if such an event occurs on a Sunday evening which, with prayers and songs, looks like a 'service of worship, to call the communication given a 'sermon.' A better word would be 'study' or 'exposition' or even 'lecture.'
By defining sermon as 'message' and connecting to a specific role within an event which is a service of worship, we are arguing that the sermon's purpose is to speak a word of God into the lives of the congregation which is heard within the medium which is itself a message, a worship service in which we celebrate the grace of God to us.
Some preachers have the ability to deliver a message in this context which takes, say, 30 minutes and some can barely sustain, say, 3 minutes. That is fine. But the quality of the message is the thing we value, and it is not measured by time.