According to Barry Collins of PC Pro News Facebook is in trouble for not deleting old user accounts. It seems that when someone quits Facebook their information is still kept.

This is good for those of you who quit and then decide that you didn't really want to quit. But if you want to quit because of a privacy issue, well then it's not so good. Your information will still be there, even though you want it gone.

I agree with Facebook keeping the information, at least for a little while. I'm going to say that I think 2 months is a good amount of time. After that you can be pretty sure the person is not coming back. Besides, Facebook must put a lot of server space into keeping all that information, they'd probably be better off deleting it.