Check the following cron task:

cat /etc/cron.d/zevenet
/30 * * * * root /usr/bin/find /tmp/ -name "cgisess_" -mmin +60 -exec rm -f {} ; &>/dev/null

Every 30 minutes this command is executed and deletes the sessions with more than 60 minutes without any activity. So change the 60 (in minutes) to the desired value.

Regards