How do I deal with a house guest who has stayed too long?

It sounds as though everyone involved in this situation is a pretty decent person, so talk to your boyfriend and explain just what you have said above, then sit down together with the guest and crack a beer. Just tell the truth: We've really enjoyed having you here, and it's been great for X to have you around. As you know, you're staying in my study, and I'm starting to find it a bit hard to get on with my work as effectively. We've really appreciated that you recognise how frustrating it's been for me, and you've been a great house guest - we'll give you a good reference! We'd really love it if you could find somewhere nearby to live and it'd be great if you could let us know when would be a realistic date for you to move out. We can help you find a place and move, and we'll be the first guests at your housewarming party! So, when do you reckon...?