Expect iPhone 8 Pre-Orders at Midnight
Again, anything is possible, but you should prepare as though you'll have to stay up until midnight (if you live on the United States' west coast) to pre-order a new device.
Apple traditionally puts its new iPhones on sale a couple of days after the media event. Last year, the iPhone 7 and iPhone 7 Plus went on sale on September 9th, two days after the announcement. We'd expect a quick turnaround to happen this year as well.
At this point, it's safe to assume iPhone 7s pre-orders will start on Friday, September 15th.
As for the pre-order time, Apple and its retail partners typically start taking iPhone pre-orders around Midnight Pacific. If you live in a different time zone, your iPhone 8 pre-order time would obviously be different.
If you want a new iPhone ASAP be prepared to stay up late or wake up early to pre-order a new iPhone. If you can't, but still want a new iPhone as soon as humanly possible, you'll probably need brave the lines at your local retailer on release day.