5 lines of code per 10 minutes is a good average.
That may seem crazy, but I'm talking averaging over a month or a year. And of course I'm talking Perl - for C multiply by 10 or 100. Plus some people code in their sleep so average over the entire 24 hour period.
Today was a good example. I wrote 3 lines of code the entire day because I spent the rest of the day looking up documentation and reading mailing list archives and working through problems in my head. So it puts my averages down. Other days I code more, or I wouldn't hit Matt's Rule.