Hi,
When you set a Maximum Security Timeout value within "Desktop Items Only" it still defaults to 2 minutes on the device but the user can change it to the maximum value that you set (It is also recommended to set "User Can Change Timeout" to True).
If you want the default timeout to change from 2 minutes to another value (e.g. the maximum timeout you set above) you need to do this from the "Password Policy Group" as seen below: