News media say that it's from 6-00 pm today, 16th sept.
If it's similar to Windows 10 updates, you know all about them on a (Tuesday) on day of release, but they don't appear here in UK until 9-30 pm, onwards.
iOS 9 downloaded today.
Start Time 19-00 hrs, end time 20-00 hrs.
The setup is biased towards Apple general policies and preferefences, but you can ignore these if you wish.
Work it out for yourself!