By default, Windows Defender automatically purges items it identified as a threat after 15 days. This preference as well as many others can be directly configured using PowerShell. Just make sure you launch PowerShell elevated.
This example queries the purge interval, then sets a new purge threshold and verifies it:
PS> (Get-MpPreference).ScanPurgeItemsAfterDelay 15 PS> Set-MpPreference -ScanPurgeItemsAfterDelay 10 PS> (Get-MpPreference).ScanPurgeItemsAfterDelay 10