15,000 ft to 30,000

Traveling at altitudes of 15,000 feet or higher above sea level is taxing for a creature that needs to breathe, because of the reduced amount of oxygen in the air. Each hour such a creature spends traveling at high altitude counts as 2 hours for the purpose of determining how long that creature can travel.
Breathing creatures can become acclimated to a high altitude by spending 30 days or more at this elevation.