Ultimately, this depends on your Apache configuration. Look for **CustomLog** directives in your Apache configuration.
A typical location for all log files is **/var/log** and subdirectories. Try **/var/log/apache/access.log** or **/var/log/apache2/access.log** or **/var/log/httpd/access.log**. If the logs aren't there, try running **locate access.log access_log**.