Can't speak to USAFA, but at Navy each case was unique. I know of probably 6 or so turn-backs during my time. Most were longer issues to deal with like cancer and they took a full year. Some even took classes at their local community college to keep classroom sharp and not lose anything while out. For those who took a whole year, especially during doolie/plebe year, when they came back they went to a new company. The one Mid I know who took only one semester had created a schedule that allowed him to pick up second semester and took 2 blocks of summer school. They then had to pick up an extra class over a few semesters and I think they took one more block of summer school another summer. They did this because they wanted to graduate with their class, but they had to work really hard, gave up some leave and were dedicated. You really need to sit down and ask alot of questions of your chain of command, admissions, academic adviser, and anyone involved in you leaving and coming back.