Marc Eisenstadt has analyzed eight years of email stats so you don't have to:

The totals in Table 1 tell me that the subjective 'quantum leap in spam' in 2002/3 that led me to install SpamAssassin as a full-time companion is certainly corroborated by the numbers. There's simply no other way to cope with the large volume of junk. But now (auto)strip away that nasty spam, and we're still looking at some scary numbers. Let's call the emails that are left over, after stipping away the nasty spam, "OK emails" (let's face it, they are never going to be "GOOD emails", right?). What we see then is an increase from 5-6K annual "OK emails" in the late nineties (15-ish daily) to 8-9K annual "OK emails" today (25-ish daily). A bright note in all this is that the numbers for 2004 are surprisingly steady compared with 2003, i.e. there's no exponential growth, even though things are clearly getting 'intense'.

25 emails daily (and thereare many I know who have WAY more than this) is a lot to deal with, especially since the emails don't cluster evenly throughout the week. To get to a 25-per-day average, you're looking at more like 30-40 per working weekday, if you're the kind of person who switches off at the weekend (ha!). If each email requires 3 minutes of thinking/response time (you're lucky if you can average that), then you've got a guaranteed two hours straight down the tubes every day.

Read the whole thing. He's got tables and everything. The comments are also interesting--tech people apparently save everything.