I've successfully changed the timeout for the portal via these steps:
Edit the settings.pcm file
Open C:\Hosting\WebApp\configuration\settings.pcm in notepad or your favorite text editor
Edit the line "<IdleTime>5</IdleTime> to whatever number of minutes you want
Edit the line with <IdleMessage> to reflect the new timeout value (this isn't necessary, but it keeps the message the user gets consistent with the actual timeout)
Save the file to your desktop, as it won't let you save to this location.
Move the newly saved file from your desktop to the location mentioned in step 1
Recycle the IIS app pool to have these new changes go into effect (or just restart the server)
Open up IIS Manager
On the left hand side, expand the tree for your server and select "Application Pools"
On the right hand pane, right-click the application pool that corresponds to the pcmon website and select "Recyle..."
The website should now follow your new timeout values.