Typically for destination weddings anywhere from a year/year and half is a good zone to send these out. You don't want to be soooo far out that people forget about it. Save the Dates are also a great way to direct your friends and family to your wedding website, where you can post updates on events, travel information, transfers, etc! But anytime in that timeframe is fine really and it does give them time to save their pennies!
Invitations typically go out 3 months before the wedding, but a lot of people are choosing only to do a Save the Date with their destination wedding these days, both to save on costs, and to save on paper/waste for the environment. If you're not doing a formal invitation, and are sending out your Save the Dates more than a year out, plan to have some kind of a reminder to your guests that can go out about 7 months prior, and perhaps another one 4 months prior. Even if it's just an email blast or evite you'll need to remind people about booking, and you'll want to get them excited about their trip!!