I've got a batch bubbling right now in the first brew bucket I purchased in 2001.
I've also got a stack of buckets. They come in handy for too many reasons to list. Any excuse to get one is a good excuse.
Right now I've got a pickle bucket with salt in it that I didn't use last winter, another with bleach solution and bottles from the redemption center. Another has starsan and a well fitting lid. Two more have batch number 63 bubbling away ( I made a double batch but only added 2hrs to the brewday, it's on another thread). I have my old bottling bucket and another with a spigot that I use with a third that is drilled with holes as a lauter tun. That original bucket has number 62 in it.
I did get a bucket, lid and adhesive thermometer my last trip to the supply store.
That first one is starting to smell rank enough to find a new use.
So to answer the original question... when it smells like ass a bucket that has had spoiled fermenting barley juice sitting in it for months if not years.