All army personnel, enlisted and commissioned officers, must meet certain requirements before they can be promoted. The most significant requirements are time in service and time in the present rank.
Recruits can join the Army as E-2s if they have 24 units of college credit. Otherwise, E-1s can expect a promotion to E-2 automatically within six months unless stopped by their commnader. Promotions up to the E-4 level are decentralized, and thus are based entirely on the decision of the unit commander (Ref 1).
It is also possible to join the Army as an E-3 if you have 48 college semester hours. Otherwise, you would need four months time in grade (TIG) as an E-2, and twelve months time in service (TIS). (Ref 2).
Though the ranks specialist and corporal both share the same grade (E-4), there is one crucial difference: Corporals are considered junior noncommissioned officers (NCOS) and thus serve in supervisory positions such as section and squad leader. Specialists do not have NCO authority. Enlistees with bachelor's degrees can join the Army as a specialist but not as a corporal. Without a degree, a private first class would have to serve six months in his current rank and have two years of total service in the Army (Ref 3).
These NCO ranks have a slightly different promotion process, called semi-centralized promotion. Unit commanders select the top performing soldiers and recommend them for promotion. However, the final promotion decision comes from a centralized board at the Department of the Army headquarters. Specialists who have eight months TIG and three years time in service are eligible for promotion to sergeant. To move on to staff sergeant, a sergeant must have ten months TIG and seven years TIS (Ref 4).
The promotion process for these ranks are completely centralized. There are only TIS requirements. A staff sergeant must have six years TIS before becoming eligible for promotion to sergeant first class. To achieve the rank of master sergeant, the TIS requirement is eight years. To become sergeant majors, soldiers must have nine years TIS (Ref 5).
Officers start their careers at the rank of second lieutenant. To achieve a second lieutenant commission, candidates are required to have a bachelor's degree and they must have completed one of the Army's three commissioning programs: the United States Military Academy, Reserve Officer Training Corps, or Officer Candidate School. To move to first lieutenant, second lieutenants must have 18 months' time in grade and time in service (pg 10).
To move up to the rank of captain, first lieutenants must have two years of experience in grade and four years total in the Army (pg 10).
Captains are eligible for promotion to the rank of major after three years in their current rank and ten years total in the service (pg 10).
Majors must serve in their current rank for at least three years before becoming eligible for promotion to lieutenant colonel, and have a total sixteen years of Army service. To move up to the rank of colonel, sometimes referred to as a "Full Bird Colonel" because of the rank's eagle insignia, leutenant colonels need to have three years' time in grade and 22 years time in service (pg 10).
There are four levels of generals in the Army: brigadier general, major general, lieutenant general and general. Generals are nominated for promotion by the president and that nomination is then confirmed by Congress. There are no time in grade or time in service requirements. However, before a colonel can be promoted to Brigadier General, he must have served in a Joint-Duty assignment, an assignment composed of personnel from two or more military services.