It depends what skulls you have on, and how well you are. For me, A hour and a half.
Create a game type that gives you the highest possible health, shields, power, lives, and whatever weapons you want. Also, set your enemies to have the lowest possible health, shields, and power. Really, all that matters is that you can never lose and that you can kill enemies quickly and efficiently. You may want to turn on the invincible to assassinations thing for yourself, too. And set it so the game is never-ending.