This post at Mere Inklings is an interesting meditation on the length of a sermon, noting that in a number of churches the pastor expects to go about half an hour or as long as 45 minutes. This happens primarily because those churches follow a Sunday morning worship model that sees the sermon as filling the same role as an academic lecture.
Personally, my sermons usually roll into the finish line at about 20-25 minutes. I can't promise my congregation that I'll never preach a longer sermon, but I do promise them that I will end my sermon when I have said what I think God has called and led me to say. And usually that's about 20 or 25 minutes, although in different settings I will preach for a shorter time.
The sermon is not the most important part of Sunday morning; it is one piece of the experience of worshiping God together. Some Sundays a line from one of the hymns matters much more than my words. Some Sundays the Scripture itself speaks much more than I do. So the idea that if I don't preach for a proper amount of time my people will never develop in their faith journeys doesn't have a very solid foundation as I view church. I'm more interested in making my sermon good -- and sometimes, that means less is much, much more.