This was posted as a reply in another post [1]. It is so important it seems good to place it in a dedicated topic.

”I'm a bit late to this thread, but I thought I'd drop my two cents. Some incredibly insightful thoughts here on this topic -- a topic I consider to be one of the most challenging aspects to baking sourdough (and all bread, I'd assume): determining when to call bulk fermentation quits. Doc.Dough had a high point at the outset: how do we measure this quantitatively? There are so many conditions with each batch of dough it's hard to compare results baker-to-baker because we're all using a different flour, different starters, different mixing methods, vessels, and so much more. Because of this, when I talk to other bakers or do a post to The Perfect Loaf sharing a formula I'm working on, I don't specifically list a percentage rise to indicate when bulk fermentation is finished. Instead of using rise as an indicator, I like to judge when bulk completes through other signs:

1. a significant rise in the dough from the beginning of bulk to the end
2. a smoothing of the dough's appearance
3. increased elasticity when tugging on the dough
4. domed edges between the dough and the bulk container
5. other signs of significant fermentation: bubbles on top and sides, and if you're using a clear container, bubbles on the bottom as well

So, many of the above are quite nebulous, but there's enough between all of those, plus experience with a particular dough, to give me a ballpark for when to end bulk fermentation.

Regarding #1: I say "significant" because I want to see some rise in the dough indicating fermentation and dough strength (a weak or overly wet mixture won't ruse much), but I don't measure the percentage rise explicitly. Why? In my experience, it can lead to false conclusions, especially if you're switching formulas frequently (which I am regularly doing). For example, a highly hydrated 100% whole wheat dough won't rise to the same height as a mostly-white formula. Therefore, saying 50% rise and trying to use that yardstick for both doughs is like comparing apples to oranges. One caveat here is if you are doing the same formula day after day, in this case, you can likely conclude percentage rise.

Numbers 2, 3, and 4 above all point to dough strength, which for me, is the number one indicator overall. As we know, a dough is strengthened not only through mixing/kneading/stretch and folds but also through fermentation as acids created as byproducts have a strengthening effect on gluten. It's easy to see this: observe your dough through the course of bulk fermentation: at the beginning, it's shaggy, sloppy, loose, and at the end, it's smoother, silky, and elastic.

Domed edges between the dough and bulk container sides further indicate strength as the gasses produced during fermentation push the dough upwards, and the center tends to dome slightly toward the outside. I also wanted to state that for me the final dough temperature (FDT) sets the stage for bulk timing. If I'm working on a mostly-white dough with typical levain percentages (10-20%), I can almost approximate when I stop bulk fermentation based merely on what the dough temp is after mixing. If I'm close to 80°F: I better start checking my dough around 3-3.5 hours; if I'm close to 75°F: it's likely going to push out to 4-5 hours. I'll use this as my coarse compass, and then from there, I use the numbered items above to further assess the dough in the moment to determine when to divide.

One thing to keep in mind here is the flour used in a formula and the hydration will drastically change bulk fermentation times and can throw a wrench into things. For instance, if I'm working on a 100% whole wheat dough at 105% hydration, the dough will have an almost flat surface in the bulk container all the way to divide time. Additionally, it may not rise all that much at all.

So to sum up, determining when to end bulk for me is much like what a (good?) doctor might do. I need to look at everything through a holistic lens to conclude: look at all the clues (the numbered items), any data collected (in our case formula and FDT), and experience to give my patient a diagnosis (when to end bulk).

Even with all this, there are plenty of times I still go to dump my dough container and find the bottom of the dough super, super active and kick myself for not tending to it 30 minutes earlier.

That's my approach to bulk fermentation. I know many bakers who use volume increase, and that's just fine, as is right with just about everything in baking: it's all about what you're used to and what works for you :)”