The phrase "a long lecture" is correct and usable in written English.
It can be used to describe a lecture that is lengthy in duration or content.
Example: "After sitting through a long lecture on quantum physics, I felt overwhelmed by the amount of information presented."
Alternatives: "an extended lecture" or "a lengthy presentation."
The man in the pet shop at the mall had given him a long lecture before he bought the animal.
And, indeed, he gave me a long lecture about the olive tree: its longevity, the depth of its roots.
As the teams prepared for the second half, Touré could be observed giving a long lecture to his right-back.
A long lecture he gave there focused on the danger that Ba'athist Iraq represented for regional and international security.
In the end one of her superiors came to the phone and our man had to listen to a long lecture onowhy he couldn't send feathers to Russia.
A man dropped into a Third Avenue haberdashery for an emergency handkerchief and found the proprietor listening to a long lecture by a corporal, apparently his son.
If you do it write (yes, the pun was intended), you should have created an approximately third-of-a-chapter long lecture.
I found this on YouTube, and it turns out that the video is a bunch of vaguely threatening images and a long, long lecture from Irvin Baxter, the Endtime Ministries head honcho, who looks like a balloon of old skin that is slowly leaking air and sounds like a cartoon frog who has been smoking for decades.
The sardonic commentary loops on — Kurt Vonnegut meeting Miss Manners — as Dr. Kussin settles himself down at the edge of your bed for a nice long lecture.
Her father gave her a long, stern lecture that has stayed with Morrison all her life.
Somewhere in the midst of all this, Hirschfeld delivers a long, spirited lecture in defense of homosexuality.
