As far as I know, SharePoint online Backups are performed every 12 hours and retained for 14 days.
SharePoint Online retains backups of all content for 14 additional days beyond actual deletion (after the data is deleted from second stage recycle bin).
If content cannot be restored via the Recycle Bin or Files Restore, an administrator can contact Microsoft Support to request a restore any time inside the 14-day window. Assisted support Engineers should help you evaluate the period which will be needed to recover the data.
More information about the backup & restore feature in SharePoint Online for your reference:
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