There is no specific "good" Timeout value that is applicable for UCM (or for that matter for any applications). It entirely depends on what exactly are your requirements, the end user expectation for time out sessions etc .
The steps to be done for setting this value / modifying it are :
1. Login to Weblogic console.
2. In the domain structure, click "Environment". Then select "Servers."
3. Click on the UCM server (e.g. UCM_server1)
4. Click on the "Deployments" tab.
5. Expand the option: Oracle Universal Content Management - Content Server
6. Click the "/"
7. Click the "Configuration" tab.
8. Set the Session Timeout value in seconds.
9. Restart WebLogic and the UCM managed server.
Hope this helps .