I've recently wanted to know how many requests per second where I having on a server. The server is using nginx as server and this is the way I found:
watch -n 1 'a="$(date +%d/%b/%Y:%H:%M:$(($(date +%S)-1)))";grep -c "$a" access.log'
I decrease the seconds in 1 unit as the current second could not be finished when the check is done (yes, it will fail on second 0 but... we have 59 seconds more!). You'll have to change the path to your access log file. You'll probably have to change the format of the date for other formats of access.log. Anybody knows a better way?