How long after being spayed should you wait before flying a dog on an international flight?
Most vets will want a follow up checkup in 1-2 weeks, to see how the incision looks and remove any skin sutures (if necessary). So certainly after that.
Spays, while very routine, are major abdominal surgery to remove the uterus and ovaries. Most dogs bounce back from spays remarkably quickly, because they're usually young healthy dogs. But their bodies still need time to heal and recover.
There is no definite answer to this question – it will depend on the dog, as well as the situation. I am sure there are some who would be fine to fly the day after a spay, while others may need longer to recover before flying. Of course there may be extenuating circumstances when pets have to travel by air soon after surgery.
Then there is the paperwork. Crossing international borders with pets is not easy, even with healthy pets. A vet would have to sign off on a health certificate or similar before international travel is allowed, and I would hope that most vets wouldn't sign the health certificate until the patient is well recovered. While I've never run into this scenario, I think I would hesitate to sign one less than two weeks after a spay, even if the dog was recovering exceptionally well.