Judgments Form in 100 Milliseconds, and Stick
In a widely-cited 2006 study published in Psychological Science, Janine Willis and Alexander Todorov asked participants to judge unfamiliar faces based on attractiveness, likeability, trustworthiness, competence, and aggressiveness after seeing them for as little as 100 milliseconds. Those split-second ratings closely matched judgments made without any time limit, and additional viewing time did not significantly change the overall evaluations—it mainly made participants more confident in them.
In other words: We start sizing people up almost immediately, and a longer look may only make us more confident in that first impression.