I mailed out STD's a year in advance that was also an informational booklet. I than plan on calling my family for RSVP's since they are NOT good at returning RSVP cards. Once I know who all is going, only those selected individuals will receive formal invitations like 4 months before the wedding.
I sent out 100 STD's and I can imagine about 40 will attend. I just don't want to spend more money on invitations to those already telling me that they can't go.