Same time next month.

For the first three decades of life, friendship runs on a schedule that somebody else writes. School puts you in the same room as the same people five days a week. Teams, dormitories, first jobs and shared apartments do the same, and none of it requires anyone to send an invitation. Then, somewhere around thirty, the schedule stops. People move for work, marry, have children and settle in different parts of a city or a country. Nobody announces the change, and so most people do not notice it until the friendships that depended on it have already thinned.

Very few adult friendships end in a quarrel. Far more end in a long series of messages that say “we should get together soon,” each one sincere and none of them followed by a date.

The figures suggest that this is not only a private experience. In the Survey Center on American Life’s American Perspectives Survey of May 2021, 27 percent of men said they had six or more close friends, down from 55 percent in 1990. Fifteen percent of men said they had no close friends at all, compared with 3 percent in 1990, a fivefold increase. Women reported declines as well, though the contraction among men was sharper. Nobody chose these outcomes. They are what happens when a structure that used to produce friendship disappears and nothing is built to replace it.

In 2018, Jeffrey Hall, a communication researcher at the University of Kansas, published a study in the Journal of Social and Personal Relationships that tried to measure how long friendship takes to form. He estimated that people need roughly 40 to 60 hours together to become casual friends, 80 to 100 hours to become friends and more than 200 hours to become close friends. He also found that the kind of time mattered. Hours spent hanging out, joking around and playing games counted for more than hours spent working side by side.

The implication is uncomfortable for anyone who believes that true friendship survives on affection alone. Loyalty matters, but it does not accumulate hours. A friendship that receives no hours slowly becomes a memory of one. The good news is that the arithmetic works in both directions. Three hours a month with the same people adds up to thirty-six hours a year, which is enough to keep a close friendship fed and, over a few years, to build a new one.

The most reliable way to supply those hours is a standing plan, which is a meeting that repeats without anyone having to organize it again. Choose one format, one place and one time, such as dinner at the same restaurant on the first Thursday of every month. Send one message to the group that names all three, and enter it in your calendar as a repeating event before anyone replies. The plan works because it removes the need for anyone to decide anything a second time.

It should be cheap and easy to repeat. A bar with a back table, a weekly pickup game or a Sunday morning walk will outlast any plan that requires tickets, travel or a babysitter booked two weeks ahead. When the group is scattered across the country, the standing plan can be a video call at the same hour each month, with the money saved going toward one trip together a year.

Small contact keeps the friendship warm between meetings. When something reminds you of a friend, whether a song, a headline or a restaurant you once went to together, send it to them with a line of your own. Put each close friend’s birthday in the calendar and call on the day rather than texting. A few times a year, have people over, and invite one person who is not yet part of the circle, since that is how the circle is replenished.

Starting a standing plan is easy. The difficult moment comes months later, when two people cancel, a third goes quiet and the fourth decides it is not worth going alone. That is how most standing plans end, and they rarely end with an announcement.

Two rules prevent it. The first is that the plan happens if two people show up, so you always go. The second is that nobody cancels without proposing a new date in the same message. “Can’t make Thursday, how about the following Tuesday?” keeps the plan alive. “Can’t make it, next time” quietly ends it.

None of this feels urgent in any given month, and that is precisely the difficulty. Adult friendship does not fail in a crisis. It fails in the absence of one, a month at a time, and the only remedy is a date that arrives whether or not anyone remembered to plan it.