I use alot of dedicated training fighters to raise my officers, and for them to have eng.spaces and smth like 18-24 months deployment time is a must, to make decent training trips without annoying problems (and only 18-24 months just to make it impossible to forget to return them and check the best of young officers for promotions).
UPD. I have no clue if it's effective ingame, or there is no difference in terms of commander training between active training trips and hangar rest time. It's just that I cannot make myself believing that fighter commanders can be truly trained while sitting in hangars.