It seems like all the young'uns have gotten into the whole blogging theories thing, so I thought I'd give it a shot. Well, this isn't so much a theory as an observation, but whatever.
I'm going to be talking about Phone Guy and Phone Dude, and theorize (okay, I lied about the theories) about how long the FNaF3 guard has been working, simply from the dialogues of these two unseen characters.
1. We shall note their talking styles. Phone Guy speaks very stiff and formally, although not nearly as much as his training videos. He still speaks without very much emotion, except anxiety. This is how one would speak to new coworkers, most likely, unless they were a social butterfly.
On the other hand, Phone Dude speaks very nonchalantly and casually, as if he was speaking to someone he had known a long time, or a good friend. It is easy to detect when he gets excited. ("We found one. A real one.")
2. Notice the first lines of what they say.
Phone Guy(FNaF2): "Uh, hello? Hello, hello? Uh, hello and welcome to your new summer job at the new and improved Freddy Fazbear's Pizza."
Phone Dude: "Hey-Hey! Glad you came back for another night!"
Phone guy is greeting a new employee. Simple as that. However, it's Phone Dude's line that really makes things interesting.
". . .another night!" This means you have been working there for at least a week. So Phone Dude would know who you are, and possibly a little about you. We know he isn't just talking about coming back a second night due to this line: "I promise it'll be a lot more interesting this time. We found some-some great new relics over the weekend." So we can safely assume this is on a monday.
My personal theory is that Phone Dude has known the FNaF3 guard a while. My guesstimation is about two weeks before the events of FNaF3. Based off of my experience, it takes about two weeks of knowing someone before you can speak so relaxedly (is that even a word) with them.
Let the rest of us know what you think.