As soon as bonus units unlocks in the DLC menu you can max out any unit if your willing to invest the time.
To max out a unit you need to promote them at 20 to a master class , then use second seal to switch classes again at 20. Level up again to 20 and your set for the game. Only reason to change classes again is if you dislike your current skill set.
It takes about 2 hours to level from 1-60 ( two class changes at 20 ) with fight animations turned off using Bonus unit fights.