http://awaremac.com/ http://awaremac.com/ Not quite what you're asking but it tracks how long you've been working (or using keyboard/mouse) and simply displays

http://awaremac.com/ http://awaremac.com/ Not quite what you're asking but it tracks how long you've been working (or using keyboard/mouse) and simply displays that without interrupting you at any point. If it's an hour or more for me, when I happen to glance at it, I get up and take a walk or stretch.