I think you are right on the money with putting the casual difficulty at around 20-25 rounds.
If I think about how many rounds, possibly that could be determined by selecting a difficulty level at the beginning?
How about casual (no one likes selecting difficulty titled 'easy') 25 rounds, hard 50, Ultimate 67 (level 66.6?). Names of difficulty should be related to fighting?
Perhaps difficulty named like: Amateur Boxer, King of Iron fist, Fist of steel, Hundred hand hitter? (Second one is trademarked to Tekken, I think).
Perhaps you could pull an old school game trick, fight every fodder back to back in Ultimate difficulty, fight the final boss, then reveal the TRUE END? Gotta have an ending!