Sermon length often depends upon the style of preaching being used. Thematic preaching seems to be shortest where a general theme is used and a few verses plucked to prop up the theme. Generally it's not teaching much, but it is meant to encourage.
Expository preaching is based upon a text of scripture and the preacher culls the depth of that verse. Such teaching/preaching can last for longer. 35-45 minutes is average, but it can go longer.
Call and response preaching is generally the longest. This is partially cultural in nature. The preacher has a theme, but the audience responds back as the theme is expressed. The more the response, the more the preacher calls out, which leads to more response. In this type of sermon you can go for two or three hours. My wife and I visited a church like this down in Tampa. The message went on for at least a couple hours. (I'm not sure the exact length because we left after an hour, went out to lunch and passed by the church an hour later and the lot was full.
I suspect culture and sermon preference plays a large part in length. My church is generally around 30 minutes for the message.