TL;DR: Friendship levels are fluid. Social Penetration Theory and Social Exchange Theory describe different levels of friendship and how people determine these levels. What you need to do to maintain a friendship depends on the level of the friendship. But since friendships are dyadic and interdependent, there's no set amount of time you could put in that guarantees friendship maintenance.
Your idea of grouping your friends into an onion model isn't that weird. It's sometimes referred to as an Interpersonal Continuum: a range of relationships from impersonal (you vs. a random passerby or server at a restaurant) to highly personal (the relationship between a child/parents, two lovers, best friends).
Even within friendships, such differences exist. Social Penetration Theory has come up with a way to describe the development of a friendship over time: As people disclose more about themselves, the relationship bond strengthens. Social Penetration Theory distinguishes 4 levels of friendship:
- orientation. People notice and interact with each other superficially, often in stereotyped ways. Most people wouldn't call this a friendship yet, but it's seen as a necessary stage for a friendship to develop.
- exploratory affective exchange. People have a relationship characterized by interactions that are friendly and relaxed, but where the interactions aren't very intimate. This is similar to a casual friendship.
- affective exchange. Here, the interactions between people are starting to incerase in intimacy and mutual understanding. This would be a close friendship.
- stable exchange. This is the stage where people know each other well, and can predict the other's behavior. The communication between them becomes increasingly intimate. This would be a best friend(ship).
Social penetration theory also predicts that maintaining casual and close friendships will take more effort than maintaining best friendship. 1
The second theory that you're going to need to be aware of when maintaining friendships, is Social Exchange Theory: It claims that you develop relationships that enable you to maximize your profits. It follows the equation Profits = rewards - costs.
- Rewards in this equations are any of those things that you're willing to pay a price (costs) for. As an example, to maintain a good friendship (a reward) you might have to travel a long distance (a cost)
- Costs are the things you normally try to avoid, like travelling long distances, sleeping on a lousy air-mattress, eating the food your friend cooks for you but you don't like, and dealing with their endless talk about their children.
- Profit is that which is left when you substact the costs from the rewards, in this case, you'd ask yourself whether having a friendship with this person really is worth all the things described under 'costs'. 2
So, when determining how to maintain a friendship, you're going to have to be aware of what that friendship is currently offering that you'd like to maintain, and what you'd be willing to sacrifice in order to maintain that.
It is important to realize a friendship is dyadic and interdependent: the relationship and its interactions involve of two people, you and your friend, and a friendship can't exist if only 1 side would label the relationship as such.
You can't control your friend changing, or your friend's life changing in a way that'll make them value other things than your current relationship. Maintaining a friendship from your side is nice, but do try to look sometimes whether a relationship changing is due to you or the other party.
You're not solely responsible for a friendship. Your friend is going to ask you incur costs to maintain the friendship, and you're going to ask your friend the same. In the end, either of you can decide the rewards no longer outweigh the costs, and the friendship will fade.
As for actually maintaining relationships, I found a few studies on ways to maintain friendships, and how these relate to different friendship levels. Long story short though: You're going to need two people, and you can't do this alone by determining an amount of time to invest. After reading the section below you can however hopefully realize a bit what you could focus your maintenance behaviors on, should you want to.
The first one3 seems to suggest that:
- Actual casual friends required significantly more proximity and less affection than either actual close or best friends.
- The three levels did not differ from each other in terms of amount of interaction.
- Both actual close and best friendships required little proximity, some affection and considerable interaction to maintain.
- For actual best friendships, interaction in the form of letters, phone calls, visits or gifts were necessary to maintain them.
The second one4 talks about 'friendship maintenance behaviors', behaviors people engage in to keep a relationship in existence or at a specific state or condition.
- Maintenance behaviors can be used strategically, when you realize something needs to be done to prevent e.g. further deterioration of the relationship, or routinely, without any specific intention or motivation. There's a suggestion that the underlying motivation, and perceptions of friend’s motive, for engaging in maintenance behaviors has implications for their effectiveness and ultimately the friendship satisfaction. So, if you use maintenance behaviors strategically and your friend notices, they may be less happy than when you use them routinely.
Perceiving that one’s friend is strategically engaging
in these behaviors was associated with increased alternatives and decreased satisfaction, costs, commitments, and investments.
- One study found four key maintenance behaviors for friendships: supportiveness, positivity, openness and interaction.
- Supportiveness are behaviors that provide assurance and support for the friend (supporting someone through tough times, making them feel good about themselves) but also support the friendship (let people know you want the relationship to last).
- Positivity includes behaviors that make the relationship rewarding (like expressing gratitude and being cheerful when together) and the absence of antisocial behaviors that negatively effect a friendship (like not returning messages)
- Openness includes the behaviors related to self-disclosure (I wrote more about that here) and general conversation (like having intellectually stimulating converations).
- Interaction includes behaviors and activities that friends engaged in together, like visiting each other's homes or celebrating special occasions together.
- Friendships are by their nature dyadic and interdependent. To maintain a relationship, it requires effort from both people, called dyadic matching and reciprocity. It is suggested that maintenance behaviors are things that friends do together.
- Friendship maintenance behaviors appear to be engaged in a manner consistent
with the predictions of the investment model framework (a.k.a. social exchange theory, profits = rewards - costs).
The second one is long, and refers to a lot of other research. I can highly recommend you check it out if you want to read more, I could access it via Google Scholar.
1: Altman, I. & Taylor, D. (1973). Social penetration: The development of interpersonal relationships. New York: Holt.
2: Joseph A. DeVito, The Interpersonal Communication Book, 15th edition.
3: Rose, Suzanna, and Felicisima C. Serafica. "Keeping and ending casual, close and best friendships." Journal of Social and Personal Relationships 3.3 (1986): 275-288.
4: Oswald, Debra L. "Maintaining long-lasting friendships." The psychology of friendship (2017): 267-282.