He published it in 1818/1819.
The book is two volumes in its final form, totalling approximately 1430 pages, depending on the edition, font size, etc.
Schopenhauer expects the reader to digest a large amount of dense prose, including untranslated quotes from other authors in Greek, Latin, French, etc.
Even more, in the first few pages of the work, he places several other requirements on his audience. First, he asks that they read his book twice.
Second, he tells the reader that, before starting the book, he should read Über die Vierfache Wurzel des Satzes vom Zureichenden Grunde (On the Fourfold Root of the Principle of Sufficient Reason), a book he had written five years earlier.
Third, he imposes the requirement that the have also read Über das Sehen und die Farben (On Vision and Colors), a book he had written two years earlier.
Finally, he mentions in passing that the reader should have a good knowledge of Kant’s works.
This would mean that a person would have to read around 3120 pages of Schopenhauer’s writing, plus however many pages of Kant, in order to meet the demands.
Schopenhauer claims that only in this way does the reader have a chance to properly understand him.
His demands are staggering, and his claim, if true, would probably mean that very few people indeed have ever understood him.
As an author, Schopenhauer is free to make whichever demands and claims he pleases. As readers, people are free to comply or ignore as they choose.
But what about his claim that this is the only way to understand him? Certainly, generations of university students have contented themselves with reading summaries of, or excerpts from, his earlier two works.
Can it be said categorically that these students cannot have understood Schopenhauer?
His prose is often witty and enjoyable to read, but does he overestimate the power of his own text? Is it really necessary for the reader to read every page of this in order to understand Schopenhauer?
He certainly wouldn’t be the first author to be a bit too impressed with his own writing skills. Surely the reader could still understand his ideas, even if the reader skipped over the occasional anecdote or literary quotation.
Schopenhauer often restates his main points in a variety of ways. To be sure, this can lead to deeper understanding, but in this astonishing amount of text, there are, and there are bound to be, a number of redundancies.
In any case, it is clear that Schopenhauer has no qualms about placing significant burdens on his reader. He also hints, in those same early pages, that’s aware that many copies of his book will finally be little more than decorative objects.
If it’s true that those books can only be understood by meeting his demands, then he’s correct that the books will be merely ornamental trinkets.