Honestly, I haven't done serious high-traffic testing to see if there's a magic number for KeepAlive. However, I've had enough traffic to the server on a couple of occasions that Apache would've fallen over if KeepAlive were on at all, since you can't specify KeepAliveTimeout in increments of less than 1 second.

There are also quite a few Apache-tuning articles on the web that feature "turn off KeepAlive!" in big bold letters, not to mention previous discussion on HN: http://news.ycombinator.com/item?id=2588783, http://news.ycombinator.com/item?id=1980278, http://news.ycombinator.com/item?id=1875848, http://www.kalzumeus.com/2010/06/19/running-apache-on-a-memo... ...