Windows Live ID: Development
It's highly unlikly that a user, while not returning to your site for weeks or months won't use other Windows Live services. I would think that each time a service gets used the date for cookie expire gets pushed back a little. Besides every time the user comes back to your site they should have to login in someway, with the exception of when they come back to your site without closing the browser. If you set your site cookine to expire when they close the broswer Windows Live will take care of the rest.
This is just from my limited experiance and from the logic I've gathered playing with it. So it's possible this is slightly inaccurate, but so far I haven't had any problems using this setup. If logic makes sence and you'd like me to go into more detail, I'd be happy to help.