Once your historical scan has been triggered - you can monitor the status of the scan in the Historical Scan center view. You can here by navigating to Nightfall -> Google Drive -> Historical Scans.
You will see a list of all the scans that have been run thus far.
In the example screenshot below, you can see a list of scans that have been run for this account. For each scan, you will see the following information: Date Scanned, Detection Rules, Date Range, Status, Total # of findings, and Total # of Files.
To export the data, into an Excel sheet, or to re-run the scan, you can select the following options on the right of the scan:
Downloading the Scan as .xlsx file
Re-run the scan if you feel you have resolved the findings and would like to confirm