Depends on the day. If it's a weekday, I'll probably be on the computer between 12 and until nearly 3. After that, I have to go to work until 11:30, and by then, I don't really feel like going on the computer and tend to just stay in my room and watch TV, maybe play a video game, and then go to sleep. On weekends, it depends if anyone else is home. I don't have access to my laptop right now, and won't until school starts up again, so I need to use the computer that's in the living room and since other people use it, if other people are home, they'd probably be on it. When school's back, I don't think I'd even have that time between 12 and 3, unless you count using the computer during class.
So add that together and I'm going to say I spend at least 15 hours a week on the computer.