Around 50 hours together to move from acquaintance to casual friend, about 90 hours to friend, and 200 or more hours to close friend. Hours spent working alongside someone counted for much less than leisure time.
How it was done
Two samples — 355 adults who had recently moved city, plus a student sample — reporting on hours spent with a new acquaintance and the resulting closeness.
What it means
The headline numbers travel further than they should, but the direction is solid and it matters: closeness is a function of accumulated shared time, and there is no shortcut that substitutes for the hours.
That makes a lost close friendship expensive in a very concrete way. Replacing one is not a matter of meeting someone new; it is a matter of finding another two hundred hours.
It is also the strongest available argument for maintenance over replacement. Keeping a friendship alive costs a fraction of what building the next one costs.