The most common answer, however, is either /etc/apache/conf or /etc/httpd/conf
Generically, you can determine the answer by running the command:
httpd -V
(That's a capital V). Or, on systems where httpd is renamed, perhaps
apache2ctl -V
This will return various details about how httpd is built and configured, including the default location of the main configuration file.
One of the lines of output should look like:
-D SERVER_CONFIG_FILE="conf/httpd.conf"
which, combined with the line:
-D HTTPD_ROOT="/etc/httpd"
will give you a full path to the default location of the configuration file
