Long-form content is writing that gives a subject enough space to be explored in meaningful depth.
There is no universally useful word count that suddenly turns something into long-form content.
Depending on the subject, it might be a detailed article, guide, report, white paper or another substantial piece of writing.
What matters is depth rather than length for its own sake.
A 3,000-word article filled with repetition is not automatically more useful than a focused 1,500-word piece.
Long-form content works particularly well when a subject requires explanation, evidence, examples or several connected ideas that cannot sensibly be reduced to a short post.
The format should follow the subject.
Sometimes the right answer is 500 words.
Sometimes the reader genuinely needs 5,000.