I think a "best friend" is judged not by how much time is spent between the two friends, but by the extent to which time will wither away at the bond between the two friends.
I have a friend that I rarely ever talk to, but whenever the opportunity arises or is pursued by either of us (even after months without any sort of communication) and we meet up again, we are still as close to each other as you can get with any friend.
Other friends, in comparison, cannot stand the test of time. Even if the friends in your immediate and daily life seem very close to you, if the bond between the two of you can deteriorate because of separation, then they are probably not a "best friend".
Nothing should be able to ruin the friendship. Nothing is required to keep the friendship strong, other than maybe mutual understanding of each other. The strength of the friendship is based on the compatibility and past experiences surrounding the two people in the friendship. In my opinion, if someone has reached "best friend" status, then you don't need to do anything to keep the strength of the friendship up, because you already know each other well enough that you know the friendship could not possibly deteriorate (unless they changed so dramatically that they became a completely different person, in which case, they aren't the same person that you were "best friends" with).