I had a similar situation this summer. My room mate's GF spent every day and night here in June and July, and most of August. Now don't get me wrong, she seems like a cool girl, and definitely didn't bring any animals over, but my main problems were that during the day, my room mate and I worked, now normally we would bump the A/C up to 85 or so to save on utilities, but since she was there she kept the thermostat at <70F. She kinda treated the place like a hotel, and frequently I'd come home from work during the day to find her and a few random people that I don't know sitting in the living room with MY surround sound up entirely too loud (I didn't even know that the amp had a setting called "max")
Last month I finally sat them both down and let them know that she is always welcome here, but that does not extend to her friends unless one of us is home, and also since our Utilities were ~350+ every month that she was here she would have to at least pay a third of that cost (really she probably used more like half of the utilities herself, but I didn't want to start trouble) They both were very mature about the whole thing and we haven't had a problem since. I recommend the same to you.