Each of the services have their own enlisted promotion systems. For the Army, Marines, and Air Force, promotions up to the grade of E-4 are pretty much automatic (assuming one doesn't get into trouble), based upon time-in-service and/or time-in-grade. The same is true for the Navy and Coast Guard up to the grade of E-3.
Here are the basic requirements for the "automatic" promotions:
Promotion to E-2:
- Army -- 6 months active duty & commander's recommendation
- Air Force -- 6 months active duty & commander's approval
- Navy -- 9 months active duty and commander's approval
- Marine Corps -- 6 months active duty
- Coast Guard -- After completion of boot camp
- Army -- 12 months active duty, 4 months as an E-2, and commander's recommendation
- Air Force -- 10 Months as an E-2, and commander's approval
- Navy -- 9 months as an E-2, demonstrated military and professional qualifications, and commander's approval
- Marine Corps -- 9 months active duty, 8 months as an E-2
- Coast Guard -- Six months as an E-2, demonstration of military & professional qualifications, & commander's approval
- Army -- 24 months active duty, 6 months as an E-3, and commander's recommendation
- Air Force -- 36 months active duty, with 20 months as an E-3, or 28 months as an E-3, whichever comes first
- Navy -- Based on Navy-wide vacancies within each career field. Averages 36 months active duty.
- Marine Corps -- 24 months active duty, 12 months as an E-3, and meet established score.
- Coast Guard -- Based on Coast Guard-wide vacancies within each career field. Averages 36 months active duty.
With the exception of the Air Force, which gives the same promotion percentages within each rank to each Air Force job, promotions (in the other branches) can depend greatly upon the current manning-level of your specific job. For example, if you're an E-5 in a Navy rating (job) that is overmanned in E-6s, you may be unable to get promoted, no matter how well you do on the tests or other promotion factors. On the other hand, if you're in a rating that is undermanned in your (next) rank, you can be promoted pretty fast, even if you don't do all that well on the promotion tests and other promotion factors.
In the Air Force, it's a different story. The Air Force gives the same promotion percentages to all their jobs (exception, some extremely critical jobs get an extra five percent promotion advantage). In other words, if the Air Force decides that their overall promotion rate to E-5 is going to be 25 percent, then 25 percent of the eligible E-4s in each Air Force Specialty will be promoted. This system has a major disadvantage, however -- it can easily result in one job being overmanned in personnel of a certain rank, and other jobs (or the same job) being undermanned in certain ranks. The Air Force handles this by identifying those in overmanned ranks/jobs and asking them to re-train. If they don't get enough volunteers, the Air Force will mandatorily re-train enough folks to balance out the rank structure within their jobs.
For an overview of the specific enlisted promotion systems, you'll want to read the following articles:
- Army Enlisted Promotion System, Simplified
- Air Force Enlisted Promotion System, Simplified
- Navy Enlisted Promotion System, Simplified
- Marine Corps Enlisted Promotion System, Simplified