The term can be used in so many different ways, and it means different things to different people
I'll say "She was one of my best friends from 5th grade." But that doesn't mean that "she" is my best friend. It just means that I was closer to "her" than most of the other people in my class or in other ones.
My youngest sister claims to have about seven best friends. Granted, she's known three or four of them since kindergarten (she'll be in 6th grade in the fall), but some of them she's only known for a year.
Of course, she makes close friends faster than I do. Or are they just not all that close? Do they talk about important, personal things? I have no idea.
I only have one best friend (plus two semi-brothers, who I never count when it comes to stuff like this because they're just practically family). We've known each other for almost seven years (seven once school starts). Then I have a few other friends with varying degrees of closeness. Then I have people who I am friendly with, but haven't invested the time into our relationships to really consider them "friends." I make friends slowly, and I don't make many, but I think that's good. It makes them more important to me.
It's taken seven years for us to get to the point where some people (like my sister?) get to far faster. But that's okay, as I said.
But it's not just time. I've known some of you guys in writing club for almost a year now, but I see you far less often than a given person with whom I share at least one class at school.
So where comes the point where "friend" becomes "best friend"? And can you have more than one? (Yes. Case point: The Sisterhood of the Travelling Pants)
People in books, I've noticed, either already have a set value of closeness, are forced into a situation in which they have to become close very quickly (like Hogwarts. They live together.), or some intense thing happens (like a war) so they bond faster. But that usually involves living at least in the same camp.
Books take place over a relatively short period of time. Usually a year at most. Any longer than that and there's usually a sequel. So by definition, you can't see the friendships that grow slowly. People find that stuff boring to think/read about, unless they're going through it IRL, and even then some people find it boring (clearly I'm not one of those.)
I suppose this isn't really philosophy, but I'll throw the label in anyway since this is one of those posts where I write an unplanned and not-well-thought-out and rambling semi-essay on a random topic of my choice. Most of those really are philosophy (or at least close enough.)
I now have Lego Star Wars: The Complete Saga, and Lego Harry Potter: Years 1-4. So I will go play those now.