In old version of the Physical Fitness Training Manual it use to specify a 30 day Acclimatization period. However the new FM 7-22 does not specify a period.
Soldiers may be deployed to theaters of operation that are at altitudes in excess of 3000 feet above sea level. Altitude acclimatization allows Soldiers to decrease their susceptibility to altitude illness and achieve optimal physical and cognitive performance for the altitude to which they are acclimatized. Altitude acclimatization has no negative side effects and will not harm health or physical performance upon return to low altitude. However, Soldiers with
good aerobic endurance may acclimatize sooner and perform better than those of low fitness levels. Refer to the following website for more detailed discussion on altitude acclimatization.