It's not about being tired of it, simply a case of how much people can take in in one go. Concentration, thoughts.
Think back to the last sermon you heard. What was your take out from it?
What do you remember? Could you make what you remember last twenty minutes if you were telling someone about it?
Ten minute of good sermon that people take something back and want to do something different, is better than an hour where people don't really take anything positive away.
And that isn't saying that no one takes anything away from a longer sermon, simply that a shorter one may be right for that congregation.
When I was a student, we had the main meeting and what were called "apologetic" talks, which were talks on single issues "Is Christianity a crutch for weak people?" for example. These were designed to take other people to that had questions about Christianity.
The main meeting had over 200 people, but the apologetic talks were typically less than a dozen attending.
In both my first and second year the CU president used to do an appeal from the front, asking people to come to the apologetics and bring friends.
The one in my first year did it in such a way, I felt like not coming to the main meeting either because I'd clearly failed. The five minutes while he talked about it made me just cringe and want to creep out in shame.
The one in my second year in just a couple of sentences made me want to run out of the door that minute and tell as many people as possible about these wonderful talks and get people along because they were so exciting.
Guess which one got me (and others) asking people (and sometimes succeeding) to come?
My point is that there isn't a right/wrong sermon length, so criticizing a shorter sermon is not correct. It may be correct that in your church a longer sermon is right. It is not necessarily right for the church up the road. It may not even be right for some of your congregation, but they may put up with it because of other things.
Different people need different things.
Me and dh have very different taste in sermons. From experience, if he's rejoicing in the main Spring Harvest speaker, then I probably will find one of the alternative speakers suits me better. Times I have really loved a speaker, and he's chosen to go to an alternative one.
Yes there have been ones we've both loved, and ones we've both not got on with, but on the whole we have different tastes. It doesn't mean that one is right, and one is wrong, or even the speakers neither of us have liked are bad. It means they don't suit us at that time.
I'm also reminded of a story a preacher I respect a lot told about when he was young.
He had a friend who was asking a lot of questions about Christianity but generally was very sceptical. He kept inviting him to different talks. Wonderful speakers, brilliant talks. And they'd get to the end, and he'd think, "he must be convinced by that" and turn to his friend who would say, "well, maybe, but..."
Then one time he invited him to a talk. It was dreadful. The speaker rambled, he couldn't even work out what point he was making, and it was he said the worst sermon he'd ever heard.
At the end the friend grabbed his arm and said with sparkling eyes, "now I understand what you mean. Please pray with me now. I want to give my live to Jesus."
The preacher told this story to say do not judge what God can use to convince people. He never found what it was that person had found in the talk that convinced him, but he said he knew that whatever it was came from God.
In the same way that different churches may do different worship, different ways of doing the intercessions, different ways of Bible studies etc. churches do different lengths of sermons. And they all can be right for the people attending that church at that time.