Announcement of the actual model, pre orders to shipping is usually pretty quick. Sometimes right after the announcement they update the Apple store and you can start ordering. But they'll usually have a press release in their newsroom as to when you can pick up at the Apple Store. If you want BTO definitely check the online store after the announcement. As to when you can pre-order/order. As they wait time will push back fast on BTO orders.
Depends. Mac Pro 2019 was announced at WWDC in June, shipping started in December. But I suppose that with consumer/prosumer products, there will be a smaller lag. Given the chip restraints, I could imagine an announcement in October, shipping in December ready for Christmas at the latest.