We have acceptable periods of time with which we can maintain eye contact. This usually lasts two, three seconds tops. Any longer than that and it just gets creepy. But don't make a judgement too fast, first take a couple factors into concideration.
Eye contact is a funny thing. Some people make eye contact often, some avert their eyes and avoid it at all costs. It's the difference between a sweet Southern grandma on a porch sippin' her lemonade saying hey to all the neighbors and a corporate businessman concentrating on his sudoku on a New York City subway. It's not necessarily a question of friendliness but more so a culturally learned behavior.
Back to the concern about the ackward length of time. The problem is the two people keep staring at each other waiting for the other person to look away, but no one will because they don't want to seem rude.
There is always the possibility that the eye contact is prolonged for a meaningful purpose, as if the person is trying to deliver a nonverbal message of critical importance.
So if it is not based on regional behaviors, manners are not at play, and there is no reason to maintain eye contact, then we can confiedently say that there is some creepin' going on.