From : http://www.cyberciti.biz/faq/automatically-log-out-linux-bash-user/
Q
. We have a few users
who frequently forget to log out and I'd like to force them or
automatically log out a user in case of inactivity detected. How do I
force user to be logged out automatically?
A.
BASH provides TMOUT
variable. It is necessary for security reasons to have an inactive user
logged out after a period of inactivity from Linux / UNIX ssh / telnet
session. This can be accomplished by setting an environment variable
TMOUT.
Automatically log out a user
Add the TMOUT variable to your /etc/bashrc file:# vi /etc/bashrc
Set TMOUT to 300 seconds (5 minuets):TMOUT=300
Save an close the file. Above config would automatically logout users
after 300 seconds of inactivity. Please note that this hack only works
with run level 2, 3 i.e. it will not work with GUI sessions.