How do you know when your starter has over-bubbled? It can do that you know, when all the food in the bowl has been consumed, in which case it falls and all its liquid runs out in a yellow, stanky pool. A perfectly ready-to-go sponge or starter looks like, well, a sponge. It has a bubble-pocked, slightly domed surface that seems to want to pull away from the sides of the bowl. A sponge in its prime like this has both the best flavor and the best leavening ability.
As the food in the bowl begins to run out, microbial gas production decreases and the dome starts to collapse. That causes a little ring-shaped crease to form about an inch from the bowl’s lip. It’s still okay to use, just hurry up and do it. Much longer and the sponge will fall in, and liquid will start to collect in the bottom of the bowl as the bugs’ salad days come to a crashing halt. You can still use this starter to flavor your bread, but you may want to spike your dough with a little extra yeast to make up for the loss of leavening power.