How to check when was last backup on iPhone?

Check iCloud Backup Date/Time - On your iPhone, Go to Settings-->>Apple ID (Your name)-->>iCloud-->>iCloud Backup and there you can see when your iPhone was last backed up on iCloud.

Check Computer Backup Date/Time - Connect your iPhone to your computer and check in Finder or iTunes (depending on which MacOS you are using). Only check on the computer if you know that you had created a backup there in the past.

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If iCloud is the way you like to create backups then turn that switch to ON. Turn iCloud backup ON. This will create backups about one every 24 hours. More information here How to back up your iPhone, iPad, and iPod touch - Apple Support

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Check iCloud Backup Date/Time - On your iPhone, Go to Settings-->>Apple ID (Your name)-->>iCloud-->>iCloud Backup and there you can see when your iPhone was last backed up on iCloud.

Check Computer Backup Date/Time - Connect your iPhone to your computer and check in Finder or iTunes (depending on which MacOS you are using). Only check on the computer if you know that you had created a backup there in the past.

iCloud backup date can be checked on iPhone and iTunes.

iTunes backup date can only be checked on Mac /PC

Watch the GIF below for checking it on iPhone

iTunes backup date can only be checked on Mac /PC

Watch the GIF below made on Mac (Catalina)