On a normal day at home with my kids, probably an hour and a half all up (mostly in little chunks). More if I get do some writing or editing in the evening. Bad for my eyes, I know. At least I'm not squinting at a phone screen all day since I don't have a smart phone. I have an ipod touch but that's mostly for listening to stuff, not looking at.
If you have to sit at a computer all day it's better if you get up and stretch often, and try and get out for a walk in your lunchbreak if you can, as well as a work before or after work (when I worked in an office I walked to and from the bus stop which was something at least). And make sure your computer setup is as ergonomic as possible, monitor settings, good desk and chair height, wrist wrest if you need one etc.