It seems to vary depending on the site and industry. I am seeing it mostly range between 5% and 15%. I have noticed it tends to slightly increase each week. Since I have only been monitoring for 4 weeks I think its a too early to make conclusions.
According to Google analytics for my largest website: Monday Dec 1st: 9.5% Monday Nov 28th: 9.5% Monday Nov 21st: 9.3% Monday Nov 14th: 8.3% Monday Nov 7th: 8.0% Monday Oct 31st: 2.7% The site has a fairly wide demographic and lots of users come in from google search.