What fraction of users that visit a web site with Google AdSense actually click on the ads?

It depends on a number of things -- the topic of the site, the relevance of the ads to the site, and the purpose of the audience in visiting the site. A site that gives tips on finding a mortgage will attract users actively looking for a mortgage provider -- and will most likely click on the ads. That site can enjoy high click through rates. A dating site on the other hand where visitors are looking for dates, are not likely to be interested in the ads and hence CTR can be low.