Hi Toni,
If save button works for you that is fine.
If you wanted to do it from automated code, you can save the document with this function also.
RMPApplication.save(successCallback, failureCallback)
But as precaution, I have seen some process instances that has been saved too many times causing problems, like after 200+ saves process instance cannot be updated.
I would advise you to execute save on some user action (like user click on some button, or particular field filled in), not automatically by time interval.
Best regards,
Taka