Alice, My Roommate's Boyfriend Won't Leave -- Ask Alice: Advice for Life at Home

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?