People who averaged only thirty hours together over nine weeks (the equivalent of just 15 minutes a day) remained acquaintances. To move from being a casual friend to a meaningful friend called for another fifty hours spent together over the course of three months, while those who advanced to be best friends took another 100 hours to be spent together. In effect, to make it into the most intimate category of friendship required something close to two hours a day to be devoted to the friend, day after day, for some considerable time. Friendship does not come cheap.
—Robin Dunbar, Friends, Little Brown, 2021