I think this is a difficult subject to handle. On one hand, yes our generation's 'attention span' is short, but so is their theological appetite depth (i.e Christian's). We should not pander to low/lazy expectations, neither should we assume or expect such a generation to be able to take and benefit from long/deep sermons, without training, prayer and hard work from we preachers'.
However I do think there is a limit to how deep a preacher can go, how much spiritual knowledge a man can impart in 20 mins or even 25 (please brother Boliver do not take that as a judgment on you or anyone else). The Apostle expresses a desire that God's people grow in their ability to digest strong meat and not merely drink milk and so I believe we should try and encourage and develop people's desire for the Word, and encoruage them to actively prepare to feed on the Word and to actively exert spiritual and physical energy in the listening to God's Word. (see Larger Catechism quoted above). (Heb 5:12-14)
I was recently preaching on 2 Samuel 24
2 Samuel 24:24 24 Then the king said to Araunah, "No, but I will surely buy it from you for a price; nor will I offer burnt offerings to the LORD my God with that which costs me nothing." So David bought the threshing floor and the oxen for fifty shekels of silver.
One of the applications I made was that there is a 'cost' to faithful, acceptable, worship and one of them is to pay the cost of putting away our desires and wants in worship, to pay the cost of time and energy in preparation and participation for and in worship. This is something we should encourage in our congregations. We should not however expect them just to be that way. We also observe in Malachi that one of the reasons God is angry with Israel is their attitude towards worship.
Malachi 1:13-14 13 You also say, 'Oh, what a weariness!' And you sneer at it," Says the LORD of hosts. "And you bring the stolen, the lame, and the sick; Thus you bring an offering! Should I accept this from your hand?" Says the LORD. 14 "But cursed be the deceiver Who has in his flock a male, And takes a vow, But sacrifices to the Lord what is blemished -- For I am a great King," Says the LORD of hosts, "And My name is to be feared among the nations.
Let us encourage our people to fight such an attitude that finds listening to God's Word preached a weariness!
I would also wonder about something else. While acknowledging that the pulpit is not any competitor to Hollywood in any way, I do think there is a problem when our people can sit captivated by 2 hours of a movie, yet find 30 minutes of communion with God in His Word too much. Believe me, I include myself in the same criticism. My concern is that we should fight this because I think at core there is a spiritual problem in us where this is the case.
Also be sure I am not equating strong meat teaching to length, merely saying that there is a limit to how much strong meat can be served in a short time. I still think a ideal length is about 40 to 45 minutes.
I will usually aim at an interesting, introduction and/or recap of 5-8 minutes, a conclusion of 5 minutes or so (maybe less) with the core exposition and application of about 25 minutes.
I do think we should include our personality, ability, experience etc. in the assessment as well, as has already been noted.