Currently it has to be completed in one sitting. There is no way to edit, save and come back later to complete.
Sorry for the inconvenience.
We just added the ability to distribute your form as a fillable PDF form and have all of the data submitted back to FormsCentral. Your respondents can save the PDF form locally, fill it out, save, come back later to finish and then submit the data. I wrote up a forum post that explains it in more detail: http://forums.adobe.com/thread/994199
Please give it a try and send us your feedback on it.