Notification Alerts

Macbook Pro: Catalina

Em Client: version 8.2.1478 (856f724)

I’m wanting to increse the notification/flag display time of incoming mail beyond 6000m/s. But by altering the setting up or down in the preferences>notifications area, doesn’t seem to make any difference to the flag display time. It seems to remain at 6 seconds

I’d apreciate any suggestions?

Many thanks