Apache HTTPD Log Time to serve request in seconds

You just need to add %T

An example of Log Format below:

LogFormat "%h %l %u %t \"%r\" %>s %b %T \"%{Referer}i\" \"%{User-Agent}i\" \"%{X-Forwarded-For}i\" \"%{SN-Akamai-IP}i\" %{Host}i \"%{JSESSIONID}C\" \"%{AWSELB}C\"" combined