I've been using MeeTimer's SQLite database to track my browsing. This query
select url,count(url_id), round(sum(duration) / 60.0, 2) as minutes from log left join urls on log.url_id=urls.id group by url order by count(url_id) desc limit 30;
returns the number of visits, and the total duration of visits to those sites in minutes.
Looks like I make lots of visits to Twitter, Twitter Search and Gmail, but spend most time on work webmail.