To minimize API calls (to avoid hitting Salesforce quota limits), your attachments are downloaded and backed up incrementally at a default rate of 5,000 files per day (you can throttle the number of files per backup through the Options page up to 600,000). Once all your files have been successfully backed-up, only files that have changed since the last backup will be downloaded (incrementally). For example, if your Salesforce Org has 13,500 file attachments, a backup of your attachments will complete within three days. From then on, your new (or modified) attachments will be incrementally downloaded and backed-up. Please note that increasing the API call limit can increase your backup time - so as a best practice, we recommend adjusting the limit gradually to keep the backup under 24 hours, or in other words, a backup a day.
For more information regarding common warnings and errors please refer to this knowledge article: Common Warnings and Errors.