rule of thumb is 6 weeks prior. However if its a destination wedding... then I think save the dates of some sort should be mailed out as far in advance as possible. Mine were mailed almost a year in advance - it give people a chance to plan and save and get time off from work. After you send the save the dates you should have a pretty good idea of who is coming and who isn't. If you have to have a final count by a certain date then I would count back 6 weeks from that date.
I sent/sending (we're trying to hand deliver them all) our Save-the-Date around the 1 year 6 month mark. Then We'll send Invitations at the 2-3 months mark. I pretty much added about 2-4 weeks to anything on my timeline, becasue I freak out about something being late or getting lost or forgoten about.
My save the dates are almost ready to go now, so it's 2 years out, then the invites will go probably around the 3 - 4 month mark. It's a big trip coming from Oz and takes some planning, so the more time the better for my guests I think...