A scanned document collection is only as safe as its backup — the right cadence depends on how frequently you're adding new, important scans.
For occasional scanning
If you scan documents infrequently, relying on your phone's normal device backup schedule (iCloud or computer backup, typically automatic and continuous or daily) is generally sufficient without any extra manual step.
For active document management
If you're actively digitizing a large backlog or regularly scanning important documents (contracts, IDs, financial records), periodically exporting key files to a second location — your own cloud storage, an external drive — adds a meaningful extra layer of safety beyond the device backup alone.
The core principle
Any single point of failure — one phone, one backup destination — carries risk; the classic backup guidance of keeping at least one copy in a genuinely separate location applies to scanned documents just as much as any other important digital file.
Is a device backup alone enough for irreplaceable scanned documents?
For most people, yes — a properly configured automatic device backup covers the common failure case (a lost or broken phone); an additional manual export matters most for the small subset of truly irreplaceable documents.