2-3 months is either a long time to have a winter fling, or a the right amount of time to mess shit up and make things awkward. How good is your relationship?

If it were me, and she was one of my (crazy, insane, unstable, insert mental disorder here) ex girlfriends who was staying in town for 3 months I might hang out with her for a month or two and keep things casual but still interesting if you know what I mean, and have the last month be a total fuck fest. Then it's not like you are too attached from having a sexual relationship for three months and when she leaves you can go about your life not knowing or caring about what she is doing.