Remember, you'll need to get them back in advance so you can start labeling place cards, and organizing seating. Make sure also to ask your caterer when they'll need final head counts. Typically, you should ask for an RSVP date 3 weeks before your wedding, if you sent them on time. At a minimum, send them out six weeks in advance, and set an RSVP date of 2 weeks before the wedding.
For a destination wedding, it's typical to send out a save the date about six months before, which will give your guests time to budget and hunt for travel deals. You should then send out the invitations eight to ten weeks before the wedding. Since these kinds of trips typically include more events than just the ceremony and reception, you'll probably want to ask for RSVPs three to four weeks before the departure date. That way you'll know how many welcome gifts you'll need, and how many surfing lessons and spa packages to book :)