Save-the-dates for a destination wedding usually go out fourteen months ahead of the wedding date. That is earlier than a local wedding (where six to eight months is the norm) because destination guests need runway to plan travel, block time off work, and budget for the trip. A destination save-the-date that arrives too close to the wedding produces two problems: guests miss the early airfare window, and some with firm work schedules cannot clear the dates at all.
Earlier than 14 months is fine. There is nothing wrong with 16 or even 18 months when the venue is booked two years out. The risk of sending too early is low; the risk of sending too late is real.