They will announce on the Lodestone when the timer is turned off, similar to this one: https://na.finalfantasyxiv.com/lodes...af5be1a019e25/. The only way to really tell is to check the posts.

It hasn't been shut off yet. I find the easiest way to find it is to type demolition into the search on the website and all the news articles at the bottom are arranged chronologically.