Spending too much time on the computer and internet can be bad for your health especially if you're spending all day on the computer and not getting outside for days.
However a lot of people do work from home and need to work on the computer for 8 to 10 hours per day and that's okay since they have to be on the computer to earn a living.
The same goes for an office job where you may need to spend 8 to 10 hours staring at a computer screen.
But if you're just playing on the computer then you really should get away from the computer every hour or so and go outside or do something else and gets some exercise to stay healthy and in shape.