To keep your eyes healthy and prevent eye strain it's recommended by eye doctors and health professionals to take a break from the computer every 2 hours for at least 15 to 20 minutes.
I work from home and have my home office desk and computer desk near a window and I can look out the window as often as I need to when I'm feeling my eyes straining.
I love the natural light the window brings into the home office as well.
At work in an office you should take your breaks as often as you can and even just turn around and look away from the computer screen for a few minutes is better than nothing.
If you feel your eyes straining then it's time to get away from the computer.
Sometimes I'll work on the computer for 10 to 30 minutes and then get up and walk around and go outside for awhile.
If it's too cold outside then I'll get up and go do something else in the house or just look out the window.