Sure Tom. I must warn you though, i'm sure ESRI frowns upon editing the base code of their widgets. I always create a backup copy of the file prior to editing anything in the Portal file system.
On your Portal server, navigate to Program Files\ArcGIS\Portal\apps\opsdashboard this folder.
Make a backup copy of the index.html file.
edit the original index.html file
in the head section, add this meta refresh tag, i grouped it in with the rest of the metas.
<meta http-equiv="refresh" content="1200">
Save the file, clear your browser cache and give it a try.
1200 second refresh equals 20 minutes. Even though the app crashes at 30 minutes, i was still seeing some issues with the refresh at 25 minutes. This tag will just refresh the entire page and grab a fresh token every 20 minutes. As our dashboards are crucial to operations here and are displayed on our video wall, having to F5 the dashboard every 30 minutes wasn't an option.
Hope this helps.