Session timeouts dictate how long a user is allowed to be inactive before being automatically logged out of JasperReports Server.
Tomcat
Edit <tomcat>\webapps\jasperserver-pro\WEB-INF\web.xml and change the sesssion-timeout. The default is 20 minutes.
<session-config>
<session-timeout>20</session-timeout>
</session-config>
Restart the application server to make the setting take effect.