while minimum time in grade to go from 2LT to 1LT may be 18 months according to that article, in the AF it's 2 years from graduation/commissioning date for SA grads.
ROTC grads have weird dates of rank so they might make 1LT after only 18-21 months active duty but it's at least 24 months after commissioning. (example, commission in May, come on active duty in Jan, DOR is sometime in Sep) ROTC grads are in reserve status til they come on active duty; SA grads are active duty upon graduation.
Then it's 2 years from 1Lt to Capt.
Army promotes to 1Lt after 18 months.
You’ll commission as a second lieutenant, once you’ve spent 24 months on active duty, you’ll be promoted to first lieutenant. You’re time in the IRR while waiting to come on active duty won’t count towards this time. The promotion clock only starts ticking once you come on active duty. Once again, you’ll spend another 24 months on active duty and then get promoted to Captain. After that, you’ll spend around 8 years as a Captain before promoting to Major if I’m not mistaken. The current stance of the Air Force is that Major will be a “give me” rank for the most part (i.e. if you haven’t screwed up and you’re a decent officer, you’ll be promoted). However this may change in the future as the needs of the service change. During the RIF’s a couple years ago, Captains who weren’t exceptional were being asked to leave the service rather than be promoted.
Officer promotions are unique in that they are based on time in grade and not skill sets or job knowledge. Granted, you’ll still have to eventually get a masters degree the higher up you go, but there will never be a test that you have to take to promote (unlike the enlisted rank structure.)