Can you help me define the meaning of occupancy on a lease in the state of Minnesota?

I have a situation where I am trying to limit the amount of time my roommate can have his girlfriend spend the night. On my lease it says "OCCUPANCY AND USE: No person other than those listed above as Resident may occupy the Apartment without the prior written approval of Management. Resident agrees to use the apartment and utilities only for ordinary residential purposes and not in any way that would cause excessive utility use, excessive wear and tear, or damage. Resident agrees that this lease cannot be assigned or the apartment sublet by resident without the prior written consent of Management." I know that my landlord will stand up for what I say because of all the other problems we have had with this roommate. What I want to know is how often could his girlfriend spend the night at our house, by law, without it being considered occupancy? I don't want to destroy my roommate's life. I just want there to be a fair and lawful limit because otherwise I know it will get extremely excessive. Also, is there a law that states that his girlfriend cannot be at the house unless he is here? I really appreciate ANY help you can offer! I have spent hours trying to look up the laws in the Minnesota legislature to no avail.