Sometime I'm using my HP for 5-6 hours per day, but it's too much I think. And if you want to use it for more hours per day, it's really advisable to remove the battery, because after few years it will work only for few minutes.
From morning to night. I'm a computer engineering student. I use SONY VAIO VPCEB4J1E (Pentium 2.13 GHz CPU).
When I was starting my ecommerce company, I was facing my PC over 15 hours a day. Not exactly healthy, but hey, at least I don't have to worry about money ever again.