How to detect how long Apache2 has been idle?

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http://serverfault.com – How can I setup my apache2 (on Ubuntu 12.04) so that a shell script can easily determine how long the web server was idle? By "idle", I mean "has not served any requests". As this is an internal development server and not a public website, this is very likely to occur in the evenings. I thought of querying the modification date of the access log, but I don't want to fill the disk with (otherwise unnecessary) logging if I don't have to. Is there another, more elegant way? Background: This is an Amazon EC2 machine for which I'm paying by the hour, so I want to keep it running only while it is (HowTos)