My iPhone and iPad show an update and I'd like to know how long this update takes.
Is this a quick update?
You won't notice a ton of new features and changes like from iOS 6 to iOS 7, but the entire storage system that holds all your apps, photos, movies and files is changed to a newer filesystem format without losing any of your data. I believe the older devices skip this filesystem reformat (iPhone 5 for example) and they might be faster udpates.
The new APFS filesystem for 64 bit iOS hardware runs storage on iOS 10.3 instead of HFS+ which has been Apple's main format on both iOS and macOS for decades.
For most people, I would recommend the following:
If you don't normally update your apps, postpone this upgrade for two or three days. Instead, go to the App Store and apply all updates. If you haven't checked if your backups are good, please go to iCloud settings or connect to iTunes and make sure your backup is complete and recent.
If you do have recent backups and have been updating your apps in the last week, you should plan for 30 minutes to apply this upgrade since it runs a conversion step when the device restarts.
For many people, it might be best to schedule the update for the evening when the device is plugged in to power and connected to WiFi. These timing allow 5 minutes to download the installer - that’s the shortest portion of this update.
The reason it will not tell you average time because that varies on connection and wifi or cellular and can take awhile once the download tell you time that can be 30 minutes or less.
imaging and updating the iOS Device depending how many apps you have and issues with device can be an average of 15-20 minutes and if the phone is from a fresh wipe can be 15 and under minutes.
Just place you device down and let it do its thing and don't play around with the phone I also change the time out from 2 minutes to never so that there is no interruption to update.