Woof:
The idea is to replicate the aerobic demands of a street fight- intense and relatively brief. Per the desires of most of the guys fighting now, most of the current matches go about 2 minutes, and if I like what's happening in the fight I will let it go longer.
Personally I preferred a 3 minute fight; in my opinion, it allowed for more variety of expression.
Woof,
Crafty Dog