Throughly trained gladiators were expected to last several years because the training was expensive. Blood from being wounded was usually enough, as soon as a gladiator was wounded he would have to plead for mercy because his chances against a skilled opponent were now nil.

There was also a referee. On occasion a gladiator would be killed or not receive mercy from the crowd. Most gladiators showed healed wounds on their bones.

Untrained people were thrown in the arena with the expectation of death. It was a favored form of execution.

Duels to the death were rather rare[1] during the Imperial Era.

1: http://en.wikipedia.org/wiki/Gladiator#Combat

Not true, very small percentage of the fights ended with death.