There is some data from the games industry, which I would say applies to knowledge workers: http://www.lostgarden.com/2008/09/rules-of-productivity-pres... (blog post from 2008 with linked presentation pdf/ppt)
Edit: In the comments on the post there's a bit more information, not sure if the slides were ever updated:
Danc September 28, 2008 10:30 PM
Appreciate the comments on the data. I'll update this presentation so that the data sources are more clearly presented. Currently, many (though not all) of the numbers are in the reference articles (but that seems to be hard to parse)