How soon can I defer my student loans after joining the military?

I am joining the Air Force in the next couple of weeks. I have around 30k in Federal Student Loans. They will pay 10k of it off for me for enlisting. How soon can I defer my loans? Right after I sign an Air Force contract? Or after Basic Training? I was just wondering if I can defer my loans while I wait to be sent to basic, if i can defer them at all?