Ah well, that depends on what the admin of the server set-up.
default it's /var/log/* .. but maybe if you run in a openvz container, vps, or smth .. those folders could be different (weird, but ok) ..
If all your log files are in /mnt/log/*
then set a line: cron.* -/mnt/log/cron.log
in your rsyslog.conf and restart rsyslogd