I, for one, do not retire my generals until they reach maximum management and influence. However much time that takes. Then I just send them to govern my largest and potentially enriching cities on the ends of my empire. Then the general's penalties will be the most effective in curbing corruption, increasing happiness and boosting income through management. Thus, I replace my generals often, so as to give everyone a chance for the double ten, or the state of having 10 management and influence. Which is very common for me.