I'm not sure how long they will last, but B&H now has the Nikon Z9 listed as "in stock." Since B&H was the last remaining back-order status vendor I know of here in the US, this would indicate to me that Z9 inventory coming into the US has now cleared all the long-held back-orders. New shipments seem to be coming in at about "demand level." With the dealers I talk to regularly, they all are saying that they either have one or two units in stock, or if they're currently out of stock they expect their next shipment from NikonUSA to fill any orders they have or they expect to get in the near term.
So I'm calling this one at 10 months. That's a few months longer than I expected, but the Z9 has also been more popular than Nikon originally expected. I can tell from my book sales that Z9 demand is still pretty high, so expect your dealer to go in and out of stock for awhile, particularly since we're coming up to the holiday buying season.