The rule of thumb is 15 minutes of rest for every 2 hours of continuous computer use. This means that for every 20 minutes of computer use you should rest for 20 seconds, staring at a distance. This helps to reduce eye strain. Tinted lenses may also help reduce problems with glare and flicker. However, no definitive studies are available to determine which colors are the best.
Tints are highly individualized, and are prescribed most on a trial and error basis.
Dry eyes can result from working at VDT's for extended periods. The two main reason are that we blink less per minute while reading or concentrating on text-like material. Also VDT's are often placed in air conditioned rooms and condition air leads to problems with dry eye and can cause problems with people wearing contact lens. If you feel that you have CVS, you should seek help from an eyecare professional.