For international excursion fares, also known as apex fares, you must book at least 21 days in advance of the flight, and you may have to purchase the tickets at the time of the reservation. Minimum stay is usually 7-10 days, and maximum stay can range from 3 months to a year.
Fares often depend on the seasons.
If you buy a round trip ticket and throw away the other half, make the first leg of the trip the destination, since some airlines will cancel the return trip if you don't show up for the first leg. If you intend to skip ONE leg of a multi-leg flight, tell this to your reservation agent in advance, so that they can annotated your PNR to prevent subsequent legs from being cancelled. It is cheaper to puchase a one way ticket to a destination.