My wedding is 14 months out and I just sent out STD's to everyone. My formal invites will go out this november (one year away) because condos go fast in Maui so I want everyone to have plenty of time to book.
I don't really think there is a "right" answer. If I was you, I would just send out the formal invite with all the important info and forget the STD. Your guests will appreciate having alot of notice to prepare if they want to go... just my opinion