When a piece of feces produced is long enough to span from one side of the toilet bowl to the other, with sufficient rigidity to fully suspended itself above the water. This is reminiscent of a fallen tree over a gorge that is so cliche in movies.
"What happened to my toothbrush?"
"Oh, sorry dude. I made a log bridge this morning, and it won't go down when I flushed. I had to break the bridge in half with your toothbrush so it would fall into the water."