It is safe to eat? Almost certainly, especially if you bake it. Your dough doesn't contain anything that will "go bad" in 15 hours at room temperature. Many bread dough recipes containing only flour, water, salt, and yeast are left to proof at room temperature for 12-24 hours, though they generally start with a much smaller amount of yeast.
Could it "go bad"? That would only be possible if one of your ingredients was already contaminated with significant amounts of bacteria, mold, etc. If you saw weird things growing on the dough or it had a foul odor, I suppose that could be an indication that something was "bad," but none of that should be possible in 15 hours for normal bread dough, assuming your ingredients weren't contaminated to begin with. (Under normal conditions, it will probably take a few days or more at room temperature for bread dough to start actually "going bad" and growing mold, etc.)
However, the bread dough has probably "gone bad" in the sense that the bread produced will likely be inferior in flavor and texture. If you bake the dough "as is," it will likely collapse significantly in the oven and be rather dense. Chances are the dough will taste a bit odd after baking -- overly "yeasty" or "beer-like," with some "off" flavors. It won't be completely inedible, but it probably won't taste great. Personally, I wouldn't waste my time doing that.
The above answer I believe covers the specific question, but what can one do in this circumstance to "save" the dough?
It is possible to try re-kneading it for a few minutes and see if it will rise some more (re-kneading will redistribute the yeast and allow them to perhaps find more food), but that seems unlikely after such a long proof with so much yeast initially.
At this point, the likely only way to save it as bread would be to use it as a "pre-ferment" for another batch, that is to cut up the dough into pieces and mix into another batch of dough (perhaps tripling the overall batch size, while using no yeast or perhaps only a small amount). Then let proof, divide, and bake. But I personally wouldn't do this unless I were sure the dough didn't taste bad, because in your situation the dough might have acquired some less desirable flavors, and you'd be wasting more ingredients to produce bread that tastes a little "off."
If you were desperate to use the dough for something and didn't want to risk an even bigger batch of inferior bread, I'd knead the dough a bit, divide it up, and use some sort of fast cooking method, probably with some other food or flavorings where a significant rise isn't needed (e.g., pizza, flatbread, fried dough, etc.).
There are three steps in bread making commonly referred to as "Proofing"
The first step is also called fermenting (or proofing the yeast, which, I believe, is not what you are asking about).
The first rise (also called proofing or bulk fermentation) is about increasing the volume. This is the primary breeding period for the yeasts once they are incorporated into the dry goods.
It's during our bulk fermentation that the yeast does the majority of
its work, helping our dough gain flavor as ethanol and other
byproducts are produced, and gain structure as CO2 inflates our gluten
network.
The second rise, or final proof, of the dough is maturing the flavor and texture. Having risen in volume and then being shaped much of the gas created in the first raise is released, but the gluten 'matrix' is preserved. How much (or how vigorously) we 'punch down' the dough will create a bread of more fine air pockets while a dough that is barely worked will preserve some of those original air pockets and will create a bread with a more course interior. Additionally the final proof can be used to allow the loaf to take it's final form. Sometimes this is because it is in some container (loaf pan, dutch oven, etc.). It is simply easier to allow the dough to fill the pan by expanding into it. If time permits it is often beneficial to 'retard' the final rise, slowing the rate of expansion, by placing the dough in a refrigerator. This will, obviously, take more time, but the reward in flavor is (IMHO) worth it.
Proofing our loaves in the fridge (also called retarding) will slow
down their final rise, giving our loaves more flavor. Also, retarding
loaves during their final proof makes them easier to handle and score
before baking, which will improve the crumb, crust, and appearance of
our baked loaves.
There are a variety of more detailed explanations available online. This article from 'Serious Eats' is a good starting place.
Best Answer
Basic room temperature breads don't prove in 6 or 12 hours, 30 minutes to 2 hours is more typical. The overall process from start to finish including baking may be 6 hours. Sourdough breads and enriched doughs like brioche and challah can take much longer to prove at room temperature.
As for calculations no, there's no straightforward time conversion between room temperature and refrigerator proving because there are too many factors like actual room temperature and yeast potency which vary how long the bread will take to prove in other location. In home baking you prove to result, not to time, i.e. you prove until bread has doubled or tripled in size however long it takes.
Refrigerator proving really does slow it down by a very large degree, I don't have a figure for it but what I will say is that if you made a dough in the morning then put it in the fridge for 10 hours it still may not be fully risen. This isn't a bad thing, it means you have a lot of flexibility. I often make pizza base the night before then leave it in the fridge for an entire day before using it with excellent results (that won't work for all types of bread as it may be too much proving).
What that means for you is that you can be strategic with it; make the dough in the morning and then put it in the fridge to prove, when you come home if it isn't fully risen it's not a problem as you just take it out of the fridge and let it warm up on the counter until it has. Or you can make the dough in an evening, rise it overnight and bake it in the morning.