As long as it takes to answer the question completely and no longer. Length correlates with performance because thorough answers tend to be longer, not because length itself is rewarded. Padding a short answer makes it worse.
A 700-word post that fully answers one question beats a 2,500-word post that circles it.
For AI extraction, the first 60 words matter more than the total.
Judge by whether a reader could act afterwards, not by word count.