There are no definitive guidelines, however it is recommended that you try to take breaks by looking away from the screen every ten to fifteen minutes. In addition you should try move away from the computer every two hours to give both your body and eyes a break.Computer users tend to blink less which leads to them experiencing dry eyes - this is particularly noticeable if you are working in an air conditioned office which also has a drying effect. One way to overcome this is to try to remember to blink every time you hit the return key!