What is the legal definition of living at an apartment?

Ok there are these large apartment complexes that do student housing. They are run by large evil corporations that like to tax the common man. The way it works is they bill each person individually, not the unit as a whole.

3 people each pay $500 each = 1500 total
4 people each pay $500 each = 2000 total
...for the same exact apartment

With that being said, what is the legal definition of living at an apartment? Am I technically a visitor if all roommates agree to have me? Assuming that how long is a visitor allowed to stay (all agreeing)? Am I still legally considered a visitor after two days? I would think so, as I have had people stay at my apartment for that long. What makes two days legally any different than 4, 8, 50, 180, or 365 days? After all I am a visitor,,,,

(What would a lawyer say?)

In practicality, I doubt I would even be noticed considering the apartment is so big and they would not enter unless they had a reason. Just don’t be an ass and get myself caught. I will just lay low and not really use the amenities that I don’t pay for anyway. Hell I’de rather give the cash to my friends anyway over “the man”