How much will being on the computer for hours at a time hurt you?

Not any more than watching TV for hours at a time or doing anything for hours at a time that is a sedentary practice. As long as you have a real life, are getting exercise and participating in your real life don’t sweat it too much unless you are spending, literally, like 8, 10, or 12 hours a day online that is not work related. I’d say spending a few hours a day is pretty normal for a lot of people.

You can get deep vein thrombosis in the legs.

I believe that if you spend enough hours sitting in any given chair seat, your hindquarters will eventually be the same size as that chair seat.

As long as you’re taking breaks, I don’t think it will hurt you. It depends on what you’re doing with the computer. I work on mine all day long. I actually have a reminder on my phone to alert me that I need to get up and walk around for awhile. Just in case I’m lucky enough to get focused on what I’m doing (procrastination is a bitch).

Well do you plan on eating? Using the washroom? Breathing? There’s all sorts of factors.

The computer itself won’t really do anything.

Depends somewhat on your constitution.
If you are built to be calm, your eyes don’t mind focusing at the same place for hours every day, and your hands can get all the human input done without stress – then you can stay pretty healthy.

Most of us have to learn some adaptive behaviors to mitigate the wear and tear on posture, eye, hand and wrist.

I spend on average ten hours a day on the computer as I am a writer.
Time flies when I write.
I consider it me putting in an eight or ten hour day as if I was at work.
I balance sitting with outdoor activities and going to the Gym.
I like the Summer months outdoors and hike and photograph more so then.
Almost every twenty minutes or so I get up from the computer to rest my eyes and do other things.