Marc Eisenstadt has analysed 15 years of email.
… it is trivially easy to get to 2.5 hours per workday assuming a fairly ruthless, ‘one-touch’, knee-jerk email interaction regime. And worse if you deviate from the regime.
Then there are other sources of workflow: blogs, aggregator summaries, phone calls (rare, but I still allow one or two), cell-phone, text message, instant messaging (my buddy list is very large, and most of them are work-related).
Interesting that Knuth opted out of email in 1990.