It depends on what officer you are like if you a Major officer and you are in charge its about 39 years. It really depends on where you are in the army.
The answer is: 15 Years.
The best place to find the answer of similar questions like this is in AR 601-280 Army Retention Program. The answer is found by seeking the rank's "Retention Control Point"...
Roman legions were kept in garrison camps usually near the frontier or other areas where it was believed they would be needed or useful. These camps often attracted merchants and other civilians to...