In which situations do I need to use the Extensive Scan option instead of Quick Scan While using the software Stellar Repair for Exchange

Quick Scan is relatively faster than Extensive Scan. You can switch to the Extensive Scan mode, if your EDB database is severely corrupt and you do not get the desired satisfactory results using Quick Scan. Notable that the Extensive Scan option thoroughly scans the corrupt EDB file to give you more accurate and reliable results.

However, it takes a considerable amount of time for the software to finish the extensive scan process. 

Once the Quick Scan results are displayed to you, Exchange Recovery prompts you to rescan the EDB database using Extensive Scan. If you do not get the expected results, you can certainly go for the extensive scan method.