How long does the enabled "Autofill" function allow my users to retain data?

I would like to assure my users that they can work on the same form over a period of two weeks.

I believe your are referring to the Save and Continue feature.  This feature is largely dependent on the users browser.  Since data is stored in the browser, if the cache/cookies are cleared for any reason, so will the data.

For a more permanent method of allowing users to update a submission, I would recommend using an edit submission link in an email autoresponder:

https://www.jotform.com/help/40-How-to-Let-Users-Update-Their-Form-Submissions-at-a-Later-Date

This will allow users to submit the form and return to their specific submission at any time to update their specific submission.