The phrase "a lengthy lecture" is correct and usable in written English.
It can be used to describe a lecture that is long in duration or extensive in content.
Example: "The professor delivered a lengthy lecture on the complexities of quantum physics, which left many students feeling overwhelmed."
Alternatives: "an extended lecture" or "a protracted lecture".
The elder Mr. Wu extols Huaxi's splendor — and the Beijing government's wisdom and foresight — in a lengthy lecture given each morning in an auditorium packed with hundreds of tourists.
What does this place ask of you?"It also gives me a lengthy lecture on "The Traveller": "To be the traveller means you are not at home here – you know you don't belong, etc".
Instead of a pithy quote, I received a lengthy lecture on the difficulty of estimating the so-called wealth effect, the perils of drawing conclusions about individuals from aggregate statistics, the lopsided nature of wealth distribution in the United States, and much else besides.
In the early 1990s, after a lengthy lecture by Jaroslaw Kaczynski on German wickedness, an exasperated Helmut Kohl, then Germany's chancellor, ordered him out of his office in Bonn and told an aide: "Don't let that man within gunshot of this building again".
In a round-table discussion before dinner on Wednesday night the prime minister asked for the floor after Timerman had delivered a lengthy lecture on Britain's "colonialism" which began 182 years ago when the UK "expelled Argentine population and authorities from the Malvinas Islands", according to the MercoPress news agency.
QPR's defensive defiance was tested even further with 12 minutes left when Taarabt was shown a second yellow card by referee Mark Clattenburg, who had infuriated the home fans seconds earlier by declining to punish Scott Parker despite delivering a lengthy lecture after a theatrical fall.
Warren was giving a lengthy speech.
The conductor was Walter Süsskind; I don't know whether he objected, but I remember a reference, in the liner notes from the LP I had of it decades ago, to Schnabel's response to a chuckle from an orchestra member at the recording session a lengthy lecture on the historical precedent for and validity of cadenzas in a musical style different from the composer's own.
In his lengthy lecture, Mr. Castro decried the globalization of economic markets and criticized what he called a lack of effort by wealthy nations to battle disease in poorer countries.
Just over halfway through the lengthy lecture on South African law and its history of granting bail that preceded the ruling, it became clear that the Magistrate considered that the prosecution had done enough to jeopardise the legitimacy of Pistorius' bail appeal.
His most famous production is a lengthy sermon-lecture series called "Constants on the Path of Jihad," which emphasizes the global nature of holy war: "If a particular people or nation is classified as... 'the people of war' in the Shariah, that classification applies to them all over the earth".
