How long should it take to update to iOS 10.3?
The update from iOS 10.2 to iOS 10.3 is a very large update under the hood, so it's not the typical update that is a quick download and quick restart. You'll likely need 10 minutes to check backup and 20-40 for the actual update (excluding download time of the update which depends on server load and network conditions).
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.

https://support.apple.com/kb/DL1893 iOS 10.3 changes

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.

My iPhone 7 (256 GB) took 28 minutes to update
My iPad Air 2 (64 GB) took 32 minutes to update