cformsII stores form information in a database. They also have the ability for multi-page forms.
Perhaps you could have a multi-page form where every time a page is submitted it stores that information in the database along with the user id of the logged-in user. Then every time they come back they could either go to the page of them form they have not yet completed or they could have the form fields auto-populate from the database.
Clearly you will just need registered users and things to be stored in the database. I’m not sure if cformsII keeps user information from the form submissions (I know they don’t require users to be logged in to fill out the forms).
This probably isn’t functional in cformsII but I’m not sure what kind of free plugin you’re going to find that offers that. I’m not sure what kind of knowledge you have as far as being able to develop this goes, but that’s a start. Maybe you could encourage the cformsII folks to add this functionality ??