To clarify, are you referring to one-on-one duels (e.g. tournaments) or on a battlefield?

Just like any real life test of skill or athleticism, this is incredibly dependent on a large number of factors. If one combatant has a significant edge on the other in skill, or if any number of outside influences effect the combatants, the fight could be over in seconds. If two fighters are on more even footing, such as in a modern day fencing match, a fight could potentially last a few minutes.