For every 20 minutes spent focusing on something within arms length you should rest your eyes for around 20 seconds. Close your eyes or at least take long blinks to recoat your cornea. During the break, focus on something that is more or less 20 ft (6 meters) away. This may not always be possible, but getting fresh air is extra refreshing to your eyes and body as a whole.
It is difficult to take breaks throughout the course of a busy day but you'll be glad you did. Our eyes work hard for us so take care of them and you'll be more productive and you'll avoid the irritating side effects of Digital Eye Strain.