There are two ways you can specify the session timeout in your websphere server : one way is to set the session time out at the server itself and another way is to set this up at the application level. By application, I mean the deployed enterprise application.
To change the server time out open your admin console and navigate to:
Servers > Application Servers > [Your Server] > Session Management
Then set the desired session timeout in minutes. Please note that you also have options to choose not to timeout at all.
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If you would like to set the session timeout at the application level, navigate to following in your admin console:
Applications > Enterprise Applications > [Your Application] > Session Management
Then set the desired session timeout in minutes.