My lovely roommate has recently met and is seeing a partner. And as nice as this guy is, he's always at our place. He technically lives with his ex-girlfriend, so my roommate cannot go there. How many nights a week is it acceptable for him to be here? Currently it's about four (which is over the weekend), and it's driving me up the wall – I feel like I'm living with a couple, and it's my apartment. How best do I handle this?
This makes me wonder [the ancient question]: What IS the right amount of a time for a roommate's significant other to spend in a shared apartment? Three nights a week seems to me like an appropriate upper limit (although if anyone has better ideas, I'm happy to hear them).
Talking to your roommate face-to-face about this, as soon as possible, will probably be a lot easier in reality than it seems in abstract — it'll be uncomfortable, sure, but not as uncomfortable as you are already, and I bet whatever you figure out will be an immediate improvement and relief. You could mention to your roommate that while you're happy for her, and you obviously like the guy, it makes you uncomfortable (or stressed, or frustrated, or whatever it is, precisely) to have an extra person spending more than three nights a week in your shared space.
Your roommate would probably be happy for an excuse to nudge this guy to move out of the home he shares with his ex-girlfriend. Asking [her to ask] him for a general timetable might be useful, too, if "right now, immediately" isn't an option.
Oh, and if your goal is to have him spend less time there, make sure you don't suggest he contribute to rent or utilities!