How does durability decrease?

It goes down by 1 every time you use an attack and hit something. Most durability items have around 20-25k, so it really depends on your class how fast you'll use that up. Repairing is cheap now anyways though.

Ah. I tested it out and it took about 325 hits to get my maple pendant down from 99% to 98%. I guess it's not an issue unless you're a BM or corsair.