I'm not actually convinced that "better" preachers can preach longer. This fact struck me rather forcibly as I listened to Derek Thomas at TLF this year. Now, Dr. Thomas is a very good preacher. However, he is a slow speaker, with long pauses between sentences. This is his style. If he's going to say much, he has to have time to do that. And his style is very engaging. I don't think he could manage to get his point across in only half an hour. However, other speakers are much quicker in their delivery style, in which case, they can preach shorter messages, and still get the same amount of information into the sermon. For myself, I generally preach 25-30 minutes. This is due in large part to the restraint I have of doing two services (at two different churches!) with not much time in between.
Time itself (either much or little!) is no guarantee of quality, as has been said. For one thing, the longer a message is, the more difficult it is to keep it on a single point. Too little time, and one risks over-simplifying the text. All sermons should, I believe, have one main point, or else the passage being preached is too long. Too many preachers think that they have to preach the entirety of Reformed doctrine in every sermon. I find such sermons incomprehensible and disorganized. The temptation is to sound learned by expounding so much of Reformed theology in every sermon (which has the added "benefit" of lengthening the sermon). However many subpoints a sermon has, it should always have one main point. This one point needs to be followed like a laser beam.
Another difference in style needs to be mentioned here, and that is the tendency of some preachers to the big picture, and the tendency of others to the details. Presumably some kind of balance would be good here. On the one hand, I have seen preachers who want to skim everything, and they never want to get down to the details, which can often preach very well. Probably this is due to their inadequate exegetical skills and/or pitiful libraries. On the other hand, there are those Lloyd-Jones wannabes, who think that if they skip over a single sub-sub-sub point of the text, then they have failed to preach the whole counsel of God. I don't believe either position is correct. When the details are important and have a direct impact on the main point, they need to be carefully exposited. I well remember the sermon given by Eric Alexander on Romans 12:1-2, where he lovingly and in fine detail parsed the Greek words of those verses, and boy, did that preach! "Be transformed" as a (divine) passive imperative really preaches, I'm telling you. But I've seen preaching where Greek words are parsed, and nothing is gained for the expounding of the main point. It is done simply to show off. This is not helpful.
It is always appropriate to preach the big picture, even when one is preaching the details. One can never lose sight of the entire Bible, even if the entire Bible should not be preached in every sermon. This can be done by showing patterns within patterns in a suggestive rather than exhaustive (which is usually exhausting!) way.
All of this has a direct impact on how long sermons should be. I don't think that congregational expectations should be anything like a deciding factor in how long a sermon should be. One generally won't have problems with 30 minute sermons. Getting up to 45 minutes will be okay in some congregations but will stretch others (which may be a good thing!).