It's a high honour to be called someone's best friend – just look at the way characters in sitcoms like Friends and How I Met Your Mother argue over which of the gang is their best friend. But the criteria for someone fitting the role of "BFF" is hard to define.

Until now, that is, as a researcher from the University of Kansas thinks he has cracked the code.

In a new paper published in March in the Journal of Social and Personal Relationships, professor of communication studies Jeffrey Hall defined how long it takes to make a friend, and how long it takes for people to typically make their way through the different stages of friendship.

In his previous research, he found a person's brain is capable of handling about 150 friendships at once.

Hall and his colleague developed an online tool to assess how close people were to their friends. In the first part of the study, they analysed 355 survey responses from adults who had moved within the last six months and were looking for new friends.

They were asked to think of someone they had met since moving, and how their friendship had developed.

They had to divulge how close they were and how many hours they spent together, as well as where they would rate the relationship in one of four categories: acquaintance, casual friend, friend, or close friend.

Through this, the researchers could estimate the number of hours where people can level up to different stages of friendship.

In the second part, 112 KU freshmen students were asked about two people they had met since starting school two weeks previously.

The researchers followed up after four and seven weeks to see what friendship achievements had been unlocked.

Results showed it takes about 40 to 60 hours to form a casual friendship, 80 to 100 hours to be upgraded to being a friend, and about 200 hours to become "good friends."

"We have to put that time in," Hall said in a statement.

"You can't snap your fingers and make a friend. Maintaining close relationships is the most important work we do in our lives – most people on their deathbeds agree."

Young people find it particularly easy to make really good friends, Hall said. In other words, they fall for people hard.

"When people transition between stages, they will double or triple the amount of time they spend with that other person in three weeks' time," he said.

"I found freshmen who spent one-third of all waking hours in a month with one good friend."

You can't force someone to be friends with you, and of course no relationship can be watered down to numbers alone. But if you want to be best friends with someone, the best way of doing it is to spend more time with them.

"Make it a priority to spend time with potential friends," said Hall.

"If you are interested in a friendship, switch up the context. If you work together, go to lunch or out for a drink. These things signal to people that you are interested in being friends with them."

This article was originally published by Business Insider.

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