I spend more than 12 hours per day in front of computer, surfing and working. Seriously it's a bad habit, but I don't know what I would do without computer and Internet.
Yes, I do sit for more than 12 hours but I don't work all the time. Most of the time its for entertainment and few hours for work.
Not that long, but for real, you shouldn't do this for that long. Leads to illnesses and shorter lifespans. Be active!