Well, first off, you left out a bunch of pertinent information. Who owns the house? Whose name is on the lease/rental agreement? Roommate situations are always tricky.
Some landlords are explicit about the person(s) whose name(s) is/are on the lease/rental agreement and any further people allowed or not allowed to live there. And some people disregard this and try to make some extra cash and 'sublet' - whether or not subletting is allowed. There are all sorts of possible answers depending on your situation.
Some landlords have a deal where they take money from each individual living there- i.e. 5 rooms, so they rent the apartment out to 5 individuals, and each person gives the landlord X amount of dollars that the landlord worked out prorated per room/including utilities or not including. Again, not a clue what your house agreement is so this is going to be long winded to cover all sorts of scenarios (would be helpful to add this information next time, FYI).
Aside from this, I'm having a hard time wrapping my head around ANYONE paying 600 or more PER PERSON for a 2 bedroom apartment (not a house) - no idea where you live, large city, small town, yes I'm aware real estate in a lot of places is expensive - so you are trying to tell me that FIVE individuals are paying 600 or so each for a TWO bedroom. Someone is retiring early on that deal, and I can only guess it's one of the people living there who probably has their name on the lease and decided to gouge everyone else. Because no legitimate landlord would get away with charging 3,000 for a 2 bedroom APARTMENT.
Now the alternative scenario is that you explained this poorly (don't take that personally, people do it all the time), and the double room itself is 620 - so one person would owe 620, two people would pay 310 each, and the triple room itself is 605, and each person should then pay 200 each.
If there is a third person (the girlfriend) there every single day then she should be paying 620 divided by 3 so each person in the double should owe 207ish.
Anyway, without any info from you I am not going to hypothesize anymore. A hunch tells me whoever is in charge of this apartment is a ripoff artist, so you should be more concerned about that issue, and less concerned about girlfriends. Methinks it's just a bunch of people squatting, or shoved in a tin can, and one person who had the 2 bedroom to themselves was looking to make a bunch of money off a bunch of suckers. I would move. Personally. But that's just me.
On a side note, people with girlfriends or boyfriends have people over all the time in all sorts of living arrangements. If I have my boyfriend over all the time I wouldn't be telling them to contact my landlord and arrange paying partial rent. That's just not done. However, it would all balance out in the end because unless we were living together and splitting utilities and rent equally, I would be at his place as well. And if it were excessive amounts of time spent, then as an adult I would just quietly offer to put money towards groceries, utilities, toilet paper, whatever, each month if I felt I was putting that much of a strain on costs. You have friends over for dinner once a week, you don't get out your calculator and work out how many kW of energy they used or TP, or water consumption while they were in your house, right? That's silly.
On another side note, if it's in fact 7 nights a week, it possibly isn't but 5 people in a small space is crowded at best, so an extra person is noticeable, very noticeable- it is possible this feels more like a financial concern because of the actual stress of having an extra person around. If her presence was not a bother probably no one would mention anything or even think of the financial burden. We do only have one opinion/perspective out of five, so it's not fair to make assumptions, but if I were addressing this fellow, I would say maybe it's time to find a place of your own if you're in such a serious relationship that your girlfriend is staying over that much, because there are other people's privacy and space to consider. Also if this living arrangement were the only option right now, the respectable thing to do would A) chat with the g/f and decide the two of you spend a ton of time at her place half the week to give your 4 roommates a break, cause that's the adult thing to do. When she started coming around more and more and more did you (the dude with the g/f) approach your 4 roommates and check beforehand if they were ok with her spending so much time there. Hypothesis really, as I would never attempt to have an intimate relationship in that kind of living situation, because intimate relationships require some sort of privacy and personal space- not in a prudish sort of way, just adult relationships (if in fact these are 5 adults) are different than just 5 party buddies stacked up in a sardine can. I'm gonna hazard a guess and say not a lot of touchy feely conversastion goes on in your house, and you guys wouldn't dream of sitting down like adults and having a 'house rules' type conversation. But the truth is, my friend, that in the adult world, that's what you do, and if a 'friendship' with one or all of your buddies can't withstand that kind of conversation then it's time to find new 'buddies' - because in the adult world you don't avoid topics just because it is not macho to do, or makes you feel uncomfortable. If you live with a bunch of roommates and someone isn't doing their share of the chores, you sit down and have a chat about it and make it work. Life. Or move out.