Andrew Janjigian has an excellent interview Addie Roberts, author of 'Secrets of Open Crumb':

I also stopped comparing my doughs to other bakers’ doughs. I would do six hours bulk fermentation because I read somewhere that his or her bread took six hours to ferment. But then when I’d look at my dough after six hours, it didn’t look ready yet. And I would end up with under-fermented bread all the time. So I little by little I would push it 30 minutes more, 30 minutes more, 30 minutes more. And I noticed that the more advanced the fermentation was, the better fermented the bread was. To me “open crumb” [really] means well-fermented bread.

In the sourdough baking community, bakers talk all of the time about [the dangers of] over-fermentation, but nobody [really] talks about under-fermentation. I think that's why people end up with under-fermented bread—because they're afraid of over-fermentation.

You can also follow Addie on Instagram via @breadstalker_.