Fighting through three rounds is enough to have been considered beating the challenge, will grant the same rewards it used to, and is still all that is needed for the Arena Master to show up personally in the Chained Court. But why stop there? Surely the devil would not let someone go from his colosseum of blood so easily, not when the crowds demand more entertainment? Would he not much rather tempt the mortal warrior that has so skillfully slaughtered his demons, flatter his might and see if he could keep him for one more round... and another... and another...
Getting tired yet? Yes, you must be, after all this destruction, but certainly such a mighty marine could handle one more round! Each round brings out more and more enemies, each deadlier than before - up to and including arch-viles, which will obviously raise all the deceased gladiators from the rounds before! The player can quit any time he wants - and be ridiculed out of the arena by the jeering crowds - given rewards based on how many rounds he endured, including rare weapons and possibly even uniques if he withstands the horde long enough.
How about one more before you go?