That's what psychologists tell us. Shorter than that, and someone seems dodgy. Much longer than that, and someone looks like they might be a stalker or about to start a fight.
{
On average, the participants were most comfortable with eye contact that lasted just over three seconds. Looking at the distribution of preferences, the vast majority of participants preferred a duration between two and five seconds. No-one preferred eye contact durations of less than a second or longer than nine seconds.
http://digest.bps.org.uk/2016/07/psychologists-have-identified-length-of.html
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