I know that Lennie and George have known each other for a long time because they both know everything about each other.
George knows that if Lennie gets into a fight, he would kill someone cause of his strength. So George has to watch out for Lennie so he don’t kill someone. Each time they get asked about something, they both know what to say. Ex: If someone asks about what they did when they killed a animal or something, one of them would have said ; ”tell him about it”. That is a sign that says that the has known each other for a long time.