When you enlisted in the Air Force, the recruiter may have offered you multiple options to serve your country. What the recruiter didn't say -- among other things -- is that the shorter your initial enlistment, the less time you have to make up your mind about reenlisting. You can "re-up" at varying points in your first enlistment, if you're eligible, but you still have to beat a couple of deadlines.
The First Deadline
If you're eligible for reenlistment and you decide to reenlist, you must declare your intention to do so at least 120 days before your date of separation from active duty -- the day you return to civilian life -- even if you have a reserve obligation to serve out. You take care of this detail by submitting a completed Air Force Form 901, the "Reenlistment Eligibility Annex to DD Form 4." The DD Form 4, "Enlistment or Reenlistment Document -- Armed Forces of the United States" is completed upon your reenlistment. This puts the Air Force on notice that you want to continue your service.
Enlistment Length is the Key
You have to know when your window for reenlistment opens. When you signed your enlistment contract, you might have enlisted for four years or six years. You might have signed up for only two years with the National Call to Service enlistment. The last lets you serve for 15 months after training, then go to another national service option or continue for 24 months in active or reserve status. Your choice affects the earliest date you can reenlist.
Four- and Six-Year Enlistments
Assuming you're otherwise eligible for reenlistment, if your initial enlistment was for four or six years, the earliest you can raise your hand again is 12 months before you're due for separation. You only have about eight months to decide, because you have to submit the AF Form 901 120 days before the planned date of separation on your records, or your Expiration Term of Service date.
If you enlisted under the National Call to Service program, you have an initial active duty requirement of 15 months following training, plus an additional 24 months of active or reserve duty. If you choose to fulfill the requirements of your enlistment with active duty, the reenlistment requirements say that NCS enlistees must have 36 months of consecutive service on their current enlistment before they can reenlist. This means that you can reenlist as soon as you have 36 consecutive months after entering basic military training.