I just sent out my STDs this past Saturday and plan to send out my wedding invites in January 2010. My wedding is April 25th, 2010. I would suggest sending out the STDs in the next 1-2 months to give your potential guests plenty of time to plan and budget for your wedding.
I've read a lot of stuff online that says you should send out the formal invites 6-8 weeks before the wedding, but I'm not sure that applies to a destination wedding. Just my two cents.