In my opinion anytime you're on the computer or other electronic device for longer than 8 hours then it becomes unhealthy unless you really need to be on the computer or other electronic device longer than 8 hours for work.
Some people have to be on the computer at work for 10 to 14 hours and I would consider that unhealthy but they have to be on the computer or device for work.
If you're just playing on the computer, watching television using a tablet etc and not getting outside for at least a few hours a day then it's not healthy.
Some days when it's extremely hot or extremely cold outside then being inside is much better for your health but you can do other things inside the house when the weather is bad instead of just watching television or using the computer or tablet etc.