Altitude training has been an element of training for years, particularly in aerobic sports. Two schools of thought have generally emerged concerning returning to sea-level for competition. One claims you would peak 48 hours after returning to a lower altitude while the other states its 18-22 days later.
The changes one finds when training at altitude are increased use of free fatty acids (within weeks), increased hemoglobin productions (days), increased oxidative enzyme activity (weeks), increased mitochondrial volume (weeks), increased ventilation (immediate).
Clearly, if these have cross over effects when returning to lower altitudes then training at altitude would have great benefit. Timing the return becomes the great problem. If you know the competition is day X, how many days before do you come down off the mountain?
Chapman et al followed 6 elite distance runners as they completed a 28 day training camp in Colorado (2150m above sea level) and then for the next 26 days after they returned to Indiana (generally 300m above sea level). Their training while at the camp was a mixture of running at above 2000m with 2 workouts/week at 1000m.
Based on the data gathered, physiological adaptations to the lower altitude do not begin to interfere with physiological performance until after day 2, so competing 2 days after returning can work. Apparently Day 7-13 is a period of rapid adaptation to the new altitude. There is rebound effect after day 15 until day 22 where athletes will have greater endurance. Thus it appears both schools of thought have some basis in reality and more research is needed to determine which protocol should be used for which type of sport.