You need to look into the mailbox diagnostic logs for this info, specifically the ELCLastSuccessTimestamp. Here's how to get it:
Export-MailboxDiagnosticLogs vasil -ExtendedProperties
Since it returns an XML blob, you might as well cast it as xml and do this:
[xml]$diag = (Export-MailboxDiagnosticLogs vasil -ExtendedProperties).MailboxLog
$diag.Properties.MailboxTable.Property | ? {$_.Name -like "ELC*"}
which will spill out all the relevant props:
Name Value
---- -----
ElcLastRunTotalProcessingTime 169632
ElcLastRunSubAssistantProcessingTime 128305
ElcLastRunUpdatedFolderCount 15
ElcLastRunTaggedFolderCount 0
ElcLastRunUpdatedItemCount 7
ElcLastRunTaggedWithArchiveItemCount 7
ElcLastRunTaggedWithExpiryItemCount 5
ElcLastRunDeletedFromRootItemCount 14
ElcLastRunDeletedFromDumpsterItemCount 0
ElcLastRunArchivedFromRootItemCount 242
ElcLastRunArchivedFromDumpsterItemCount 0
ELCLastSuccessTimestamp 5/17/2022 7:48:23 PM
ElcLastRunSkippedNoTagItemCount 0
ElcLastRunSkippedWithTagItemCount 0
ElcLastRunSkippedNotExcludedItemCount 0
ElcFaiSaveStatus SaveSucceeded
ElcFaiDeleteStatus DeleteNotAttempted