Final answer:
The diagnostic APFT should be administered once every four weeks through the 20th week of training to assess soldiers' fitness levels and provide feedback.
Step-by-step explanation:
The diagnostic Army Physical Fitness Test (APFT) is designed to assess the physical fitness and endurance of soldiers in the military. According to the guidelines set by the United States Army, the diagnostic APFT should be administered at least once every four weeks until the 20th week of training. The purpose is to ensure that soldiers are meeting the physical standards and to provide them with feedback on their progress. The frequency of the diagnostic APFT allows for monitoring of physical fitness levels and for implementation of corrective training to address any deficiencies.