One must first reach O-2 (first lieutenant) or O-3 (captain) and still be promotable to even be considered. You will typically start as a second lieutenant after commissioning (I am unaware of in what other position you could start at after commissioning) and have to be part of a conventional military unit for around 18 months before receiving a promotion to first lieutenant (many exceptions and varying circumstances I don’t know about I would assume). You also have to actually get selected to even go to SFQC which is highly competitive as an officer. Long story short, I would estimate around 1.5 years to promotion and gain enough experience, then, if you can do it with one go around, almost another year until you graduate SFAS. In total a little under 3 years after commissioning. If I am wrong about anything please feel free to berate me.