In 2018 Jeffrey Hall, a communication professor at the University of Kansas, published a study of how friendships form. It takes roughly 50 hours together to move from acquaintance to casual friend. Becoming a friend takes about 90 hours, and becoming a close friend takes more than 200 hours.
Hours spent working together counted for less. Some adults had spent hundreds of hours with colleagues and still called them acquaintances. Hall’s advice was: “If you work together, go to lunch or out for a drink.”