I like 25 at bats for everyone that matters (BL players and prospects I feel will make the majors) there is enough to go around if you plan it right.
For pitchers, I like 5 appearances. Amount of innings is irrelevant to me. Just so long as they step on the mound.
I've heard people saying that giving vets spring training time prevents decline in ratings but I've yet to see proof for or against this.