I think the average is difficult to obtain being as there are two types of hubpages writers. And I find it easier to estimate on a monthly basis than a weekly one.
There are some people on this site who write casually, when they get the chance, and for fun. My estimate for them is maybe 10 hubs or less a month.
But there are also some people on the site who view this as a job, a way to make money, or a way to gain a reputation on the internet. They usually write 20-30 hubs a month or more depending on their real job.