How does durability decrease?

I've been looking in Southperry looking for a formula on how durability goes down. I noticed it went down without taking any damage so it's not solely damage taken. Has people tested out how it works?

I don't want my 7th Anniversary Maple Power Pendant disappear in the middle of a HT run :/

I believe it's how long you use it, not how much damage it takes.
Don't ask me how long until it hits 0, I have no idea.