RSVP etiquette

I have watched with great interest the flow of RSVP's for our wedding day. Some people respond immediately (as would be the case with Dan), others not immediately, cause they might not want to seem as if they have been awaiting the arrival of the invite in anticipation and then there are the ones that respond a day before the due date (that would be me!). We have also had some that have not responded at all, leaving us with the nagging fear that the invitations never arrived...we will be blaming the South African postal service. Going to check online what is the correct time to RSVP to an invitation...will let you know.