The 20-20-20 Rule

Mayo Clinic:

Throughout the day, give your eyes a break by forcing them to focus on something other than on your computer screen. A good rule of thumb is to follow the 20-20-20 rule: Every 20 minutes, take your eyes off your computer and look at something 20 feet away for at least 20 seconds. It’s reasonable to take a break every 15 to 30 minutes for one to three minutes. Do other work, such as phone calls or filing, during this time. Try to stand up and move around at least once every hour or so. If possible, lean back and close your eyes for a few moments.

This fits well with the Pomodoro Technique; to keep it simple, adjust the time to 20 minute intervals. You gain productivity and save your eyes.