SAVE DRAFT FUNCTION

For length of time, I do not believe there is any limit on how long a "save draft" of a task will last, as long as the process remains active.

For the attached documents, I assume (but you could test this easily) that the documents will remain in the reserved appian temporary location until the form is submitted.  But they should save along with the draft, if that's what you're trying to confirm. (edit: confirmed below that you can't save a draft for a task with uploaded documents.)

The draft stays until the task has been submitted.

AFAIK, Save Draft does not work with documents.