what does a major have to do to become a lt. colonel

A Major usually has to meet time-in-grade and time-in-service requirements, complete required professional military education, keep a strong performance record, and then be selected by a promotion board for lieutenant colonel. In the Army, this is generally a competitive promotion rather than an automatic step up, and most officers are promoted around the 18-year-of-service range.

What the board looks for

Promotion boards typically review:

• Performance evaluations.
• Leadership potential.
• Completion of required schooling or equivalent professional military education.
• Career assignments and breadth of experience.
• Medical, legal, and conduct eligibility.

Army example

For the Army, one source says a major becomes eligible after about 15 years of service and 5 years in grade, though many are not actually promoted until closer to 18 years of service. It also notes that the board selects officers based on their records, evaluations, and development assignments.

Simple version

In plain terms: a major has to serve long enough, perform well enough, and be selected. That usually means strong evaluations, the right education, and no disqualifying issues.