How do I do this?

I have an active survey that ends in about two weeks. I have set it up so that participants may complete it in multiple sessions from the same device but any survey not submitted in one week will get deleted. My question is, what do I need to do to allow me to save surveys that are almost complete (say, completed all substantive questions and most of the background questions) but not yet submitted at the moment the survey ends. Thanks, - gg