Be careful not to accept military missions that clearly cannot be finished in time. You will have to look at the destination starsystem, estimate your travelling time through hyperspace and the time you need in the target system to reach your destination. A good distance estimate is to add 10 AU to the orbital distance of your destination. If your destination is orbiting another planet use the orbital distance of this planet instead and add the orbital distance of your destination to the result.
If you don't get missions from your local military agent, don't despair. Often it is sufficient to change the starport in the same system to get missions. If all else fails and you don't want to change the base for your military career, simply make a small hyperspace jump out of the system and back. In the mean time your military agent may have changed his mind. If he still has nothing to do for you you should ask yourself a few questions. Would you give someone like you military missions? For whom have you worked in the past? Are you really trustworthy? See, you know the reason!