Your teacher is full of it. Plenty of long reads are still popular, and penny dreadfuls/pulp fiction have always been around.
Oh, and I can never get through Ayn Rand. Not because of the length, but because she is soooooo full of shit.

The Game of Thrones series started long ago, but was popular in written form before the TV series, Harry Potter? Percy Jackson? There are many sprawling and evolving stories, I think that what has really changed is how the story is ingested, with multiple media forms now available.
That said, given the pace of life, a lot of people do want instant gratification, but I don't that's purely generational any more.

Without commenting on the subject matter, books of over a thousand pages have been the bane of publishers from their inception. Huge epic novels present production problems that publishers would rather avoid. It is not the reader, but the producer that dictates that novels be more modest (less than 500 pages). Epic novels are only ever printed for authors that have proven sales records. Neither Rand nor Tolstoy published their epic novels(over 1000 pages) as their first work. Writers write what their publishers tell them too, publishers tell writers to write what will sell today. Don't forget that Lord of the rings was not a single novel, but 3.