We send out our STDs about a year in advance. We were really lucky because pretty much everyone RSVPd (even thought we didnt ask for one) so we have decided to forgot formal invitations. However, we were going to send them out about 6 months in advance. That way they have the information and all the necessary information to make their decision and book. I dont think there is a right or wrong time, it depends on the time of year you are getting married and your destination.