To see when an NCR has been marked as closed you can use the Audit history to see the date it was changed; to bring up the Audit in this case there are a couple of ways:
The first way would be to go to:
Your Supplier and Rejects Maintenance window;
From there press Ctrl + A on your keyboard;
Then simply search for the appropriate NCR.
The other way to get there would be to:
Press Ctrl+A on your home screen to bring up the Audit menu;
click on Quality to expand the options
Go to Supplier Quality
And then click on Supplier Reject
Search for the appropriate NCR