It is rare that I see a speaker -- whether a minister, priest or a person giving a non-religious speech -- who can successfully pull off a 55 minute speech. You have to be a very skilled speaker to keep people's attention and give them an hours worth of memorable content. The reason the 20 minute speech is a good bet is because it includes 2 or 3 well-developed concepts that people will remember, at least for a short while.
I also give a fair number of talks and presentations in my job and the one humbling fact that you learn is that you are lucky if people remember one thing you said a week after you speak.
Aeons ago, I spent few years teaching High School. I had no trouble keeping the kids' attention for the entire class period, which sometimes exceeded 1 hour.
I accomplished this several ways ... Each lesson had a purpose, a beginning, an end, an exposition in between. I also made the lesson interactive ... rhetorical questions are pointless. If I asked a question, I expected an answer ... if one was not volunteered I called students by name.
Some ministers attempt to employ interaction; too often it elicits only choruses of "MMMM-Hmmmm" and "Amen".
Aye, there’s the key: a successful speech with memorable content. The problem I hear with most sermons is that they fail to deliver a point. Yes, they cite scripture; yes, they elucidate what Christians should do; yes, they expound on key elements of faith. What they do not do is effectively tell people how the message applies to them and their lives. People leave the pews thinking they have been educated; what they haven’t been is convicted!