Here you will find information on how promotions happen within the Marine Corps.
The Marine Corps takes the number of “slots” they have for each enlisted rank, above the rank of E-3, and allocates them to the different Military Occupational Specialities (MOS). (enlisted jobs).
In order to promote someone (above the rank of E-3), there must be a vacancy. For example, if an E-7 retires in a certain MOS, that means that one E-6 can be promoted to E-7, and that opens up an E-6 slot, so one E-5 can be promoted to E-6, and so-forth on down the line.
The promotions to E-4 and above in the Marine Corps are competitive. That means there are only so many “vacancies” in each grade (above E-3) in each MOS (job).
The Marines need a system to decide, who, within each MOS are the ones to get promoted. For promotions to the ranks of Corporal (E-4) and Sergeant (E-5), the Marines use a system of composite scores.
Warrant officers in the Marines follow somewhat of a different track than other members. They are assigned duties only to their area of expertise. “Unrestricted” officers may carry out a wide variety of assignments during their career. For advancement above the rank of sergeant, officers must apply and enroll in the Warrant Officer Program to become a permanent Warrant Officer.
Time is also a big factor in getting promoted up in rank. Private to Private First Class takes six months. PFC to Lance Corporal takes eight months. LC to Corporal is another eight months and Corporal to Sergeant lasts two years.