There is no consensus on that in international law for now, some will suggest that national airspace ends at about the height of 100km if I'm not mistaken.
As I said, no consensus as of yet, though most international law experts say the 100km range is about right. There is also another principle which doesn't take the height into account, but instead the function of the flight, that's why it's called the functional principle at least in my language, not sure if that's the proper translation in english . If the flight is meant to be performed inside the atmosphere then various national laws (plus some conventions) apply, depending on where it is. If it goes outside the atmosphere then only international law, or more specificaly, several treaties apply as the governing law.