On a network I administer some of the end users wanted the session timeout in OWA to be longer so they don’t have to keep logging in after being idle for a while. This is an Exchange 2007 environment. This setting is stored in the registry on the Exchange server with the Client Access Server (CAS) role installed. There is two ways to modify this setting. One of course is to go into the registry and add the key for this value and set this value to the amount of time you want the session timeout to be. The second way to set the session timeout is a by running a PowerShell command.
NOTE: The session timeout is set in seconds for example 1800 seconds = 30 minutes.