Generally, between each blink is an interval of 2–10 seconds; actual rates vary by individual averaging around 10 blinks per minute in a laboratory setting. However, when the eyes are focused on an object for an extended period of time, such as when reading, the rate of blinking decreases to about 3 to 4 times per minute. This is the major reason that eyes dry out and become fatigued when reading. ( Wikipedia)
This app beeps every 8 seconds, to remind you to blink so your eyes won't get very tiered when using your PC or when you are very focused on doing something else like reading.
Every 20 minutes, look at an object 20 feet or more away for 20 seconds.
This app will tell you to stop working every 20 minutes and announce you that you can start again after 60 seconds. Meanwhile you get the time to do some of the things written above.