Just being on the computer for 10 hours or longer is not bad depending on why you're using teh computer.
Some people work in offices and have to spend 10 hours per day on the computer sitting at a desk and that's not the healthiest thing to do.
But you must sit at the computer and work on the computer for 10 hours per day to earn a living.
So in that case it's kind of bad but not all that bad.
If you're just going on the computer 10 hours per day everyday for gaming and other non essential things and you hardly ever leave the computer or go outside for exercise then yes that can be bad.
We all need exercise for our health and we need time away from the computer as often as possible.
I used to be on the computer at home nearly all day when I was a kid and didn't have a job.
But now I spend only around 5 hours per day on the computer at home or when it's bad weather outside and I have nothing else to do in the house I'll stay on the computer longer.