During WWDC Apple stated that updated to iOS 9 will only require 1.3GB of free space. That's down from the approximately 4.6GB iOS 8 required.
David Smith decided to look data from users of one of his apps to what this impace may be:
The result was pretty promising.
66% of my customers on eligible devices have at least 1.3GB of free space. This compares to just 37% of users who would have immediately had sufficient space at the old iOS 8 requirement.
Nice. And that doesn't include a new feature found in iOS 9 beta which can delete and reinstall apps to make space for updates. Apple is effectively removing limited space as a barrier to upgrading.