[fullsailor]

iOS updates are available for everyone at the same time, but Apple does stagger the update notifications. So users that know an update is available can go to Settings and find it immediately, but notifications go out in stages after the release.

[Uehreka]

Sure, but for the purposes of this discussion I think it's enough to say that Apple releases it all at once. Apple stages their updates over the course of hours, maybe a few days tops. The Android+Custom Skin sort of staging ends up spacing things out over months.