There is a facility for you to set the time of inactivity before a user is logged off. You can get more information on this from the web at:
http://www.sjsoft.com/docs/jlogbook/admin/userconfiguration.htm
It is called Log Off Delay and it can be described as follows:
This is the maximum inactivity time in seconds by the user, which will result in the system logging the user off. For example, if the Log Off Delay is set to 300 seconds, (5 minutes) and the user does nothing for 301 seconds, then the next time the user attempts any action, the log-in message will appear (with the name of the user filled in).

You simply go to the user configuration site and change the Log Off Delay setting for that user.