This is not something that we currently support for HTML (web) forms but we are working on this and it should be available in the next few months.
Note that this is avaialbe already for PDF forms. If you have a paid subscription you can genereate a PDF instead of an HTMl form. Your fillers will need to use Adobe Acrobat, Adobe REader or Adobe Reader Mobile to fill out the form. They can save their work in progrress and submit whenever they are ready to do so.