Slow night at work last night so I did some research. Assuming Maverick was the Commander Air Group he would have been an 06 (Captain/Colonel). In the trailer Tom Skerritt says that he can't be promoted whatever they're going to say he did if it was so bad that he was permanently passed over for promotion they would have already been processing him for separation and he never would have had a CAG slot. Even if he just was randomly passed over for promotion to Rear Admiral (lower half) the most they would have let him stay on active duty was 30 years