Currently you can defer an iOS update from installing up to 90 days. That however is 90 days from the release of the latest iOS version, NOT from when the profile is deployed. There should be an option to have the iOS deferred indefinitely as well because if it's past 90 days from when an update is released, the user is prompted about an available update to install, thus the deferment doesn't really work at that point.
In addition, it'd be helpful to have a countdown in the profile that says how many days are left on the deferment if you've chosen a selected time frame (see image)
Lastly, if deferment starts based on an iOS release date, the deferment option (if checked off) should re-start automatically if a new iOS version comes out before the deferment period lapses.