You may not be able to limit the amount of time in front of a screen (especially if your job keeps you deskbound), but these guidelines from the American Optometric Association can help reduce eyestrain and other posture-related symptoms. If possible, reposition your computer screen to be about 20 degrees below eye level (4 or 5 inches, measured about 25 inches from the center of the screen to your eyes). Try to avoid glare from overhead lighting or outside windows by moving the screen, using blinds, or attaching a glare filter.
And give it a rest! For every 20 minutes of computer use, look away for at least 20 seconds to rest and refocus your eyes before continuing. And, after two hours on the computer, rest eyes for 15 minutes (take a walk, get coffee, etc.). And don’t forget to blink often.