I am currently going through the process of an IST right now. I can attest to the fact that it takes anywhere from 8-12 months from start to finish. I am a non-aviation specialty active Army Officer vying for a shot at SNA. However, I turn 30 next month, so I will probably need an age waiver. The age limit is 31 for SNFO, so I am good there. I should find out for sure within a week or so. Nevertheless, by definition, interservice transfers involve already commissioned officers switching services, and they are quite possible. Each service has its own regulation governing the switch. The common denominator among them is that you must first get released from the parent service, and then get picked up by the gaining service (this is where I am right now). Check out SECNAVINST 1000.7D (Navy), USAF Reg 35-39 (Air Force), P1100.73 (Marine Corps), or AR 614-120 (Army) for guidance on what to submit in your packet. Hope this helps.