Quick Advice: On average, look straight into someone’s eyes for 3-4 seconds for every one second break.
Why Is This Important? If you don’t make enough eye contact people might think you’re aloof or insecure, if you make too much people find it creepy or could think you are threatening, so how much eye contact is expected?
In a study by Alan Johnson and his colleagues, they had participants watch video clips of people who appeared to be looking at the participants. They varied the amount of eye contact between videos and asked participants to rate how comfortable they felt during each clip.
The results suggest that the average subject was most comfortable with 3.3 second of eye contact from others. In practice, look directly into someone’s eyes for 3 to 4 seconds, then break your gaze for 1 or 2 seconds before making eye contact again.
If you want to practice in the privacy of your home (don’t use it in real interactions), I would recommend downloading a Tabata timer app for your phone. It’s a timer for exercise designed to repeatedly alternate between bursts of exercise followed by short rest periods.