Seriously though? In basille's I can't even do the run from the second bonfire to the bosses, before my ultra greatsword goes from 100% to 20% durability. And during the boss fight, I need to burn through 2 other strength weapons.
An Ultra-Greatsword when you are in the Bastille?
Let me guess, you picked up an Old Knight Ultra Greatsword. The entire Old Knight set, weapons and armor, all have extremely low durability. I suppose the idea behind the design was to place decent weapons and armor in the early game (Heide's Tower can easily be the first real area a player visits) with low durability as a balancing cost.
Most Old Knight stuff has 20 durability points. The typical Dexterity weapon has 30-40 durability and Strength weapons have 60+ durability.