I didn't even read the "a week" part and voted on my daily usage of the site which is usually about 2-4 hours (usually in one sitting) but depending on the day can totally exceed that. And i usually go on multiple times a day too!
Uh... it varies big time & depends on how many hours I've been working that week. Lately, I've been on less. I wasn't on at all yesterday because I've been busy with kids and work, but I usually spend quite a few hours here every week.