A tough nut to crack to be sure, but Randall Munroe has taken a stab at it on his wonderfully quirky What If? site.
"The average person can read at 200-300 words per minute. If the average living writer, over their entire lifetime, falls somewhere between Isaac Asimov and Harper Lee, they might produce 0.05 words per minute over their entire lifetime. If you were to read for 16 hours a day at 300 words per minute, you could keep up with a world containing an average population of 100,000 living Harper Lees or 400 living Isaac Asimovs."