My question is, how do you know when an enormous project like this, conducted over an 18-month time span is "done"? I assume you get a lot of leeway from editors and publishers on this matter. How do you make the decision to finally pull the trigger on publishing?
You try to reach a critical mass of detailed, rounded understanding of a central question, integrating the most meaningful perspectives, interrogating the weak points and blind spots, and backing up the assertions with documentary evidence or strong sourcing. Eventually, you reach a point where enough sources and materials are reliably triangulating toward the same truths.
As you guessed, there's external pressures that figure in this analysis—whether competitors are closing in on the same leads; what's happening in the broader news cycle that might make a story feel more or less relevant. As you also guessed, I am more fortunate than most writers in the degree to which I get to hold off until something feels fully baked. Mostly, writers simply have to hit a deadline, and resources run out before ambition does. I have deadlines and constraints too, but I get a lot of say in how I organize all of the above.
Then there's the actual process of creating the story. Writing a densely evidence-based investigative piece is labor-intensive—in this case, weeks of initial drafting, and then much iteration. The fact-checking process at the New Yorker is exhaustive, and can span weeks. Every sentence, assertion, and piece of underlying sourcing get scrubbed by multiple independent pairs of eyes. This story had four fact-checkers working on it for the better part of a two week period, pulling very long hours. This is all brought together in a closing meeting where each sentence is revised and polished in a group.
This is all done as additional information comes in—in fact, with these large-scale bodies of reporting, there is very often a snowball effect, where a lot comes in at the end.