You can change it at the server via web.config, but it is a very, very bad idea.
<configuration>
<system.web>
<sessionState timeout="20"></sessionState>
</system.web>
</configuration>
However, this will tie up server resources until the session ends, which may reduce the number of users you can support, or badly affect the response time to users.
Think what would happen if you do set the session so that it lasts until the user logs off. What would happen if the user doesn't log off, but gets disconnected, say? The resources are still tied up at the server...
Instead, think about using cookies instead of session variables - they are intended to be persistent, which sessions aren't.